diff --git a/libs/ardour/ardour/midi_source.h b/libs/ardour/ardour/midi_source.h
index 364b220ebb..65e382c4b7 100644
--- a/libs/ardour/ardour/midi_source.h
+++ b/libs/ardour/ardour/midi_source.h
@@ -140,6 +140,11 @@ class LIBARDOUR_API MidiSource : virtual public Source, public boost::enable_sha
 	virtual void load_model(bool lock=true, bool force_reload=false) = 0;
 	virtual void destroy_model() = 0;
 
+	/** This must be called with the source lock held whenever the
+	 *  source/model contents have been changed (reset iterators/cache/etc).
+	 */
+	void invalidate();
+
 	void set_note_mode(NoteMode mode);
 
 	boost::shared_ptr<MidiModel> model() { return _model; }
@@ -186,7 +191,11 @@ class LIBARDOUR_API MidiSource : virtual public Source, public boost::enable_sha
 	boost::shared_ptr<MidiModel> _model;
 	bool                         _writing;
 
-	mutable double _length_beats;
+	mutable Evoral::Sequence<Evoral::MusicalTime>::const_iterator _model_iter;
+	mutable bool                                                  _model_iter_valid;
+
+	mutable double     _length_beats;
+	mutable framepos_t _last_read_end;
 
 	/** The total duration of the current capture. */
 	framepos_t _capture_length;
diff --git a/libs/ardour/midi_model.cc b/libs/ardour/midi_model.cc
index 8e17e1d3ec..259a04bc0f 100644
--- a/libs/ardour/midi_model.cc
+++ b/libs/ardour/midi_model.cc
@@ -1630,6 +1630,7 @@ MidiModel::edit_lock()
 	assert (ms);
 
 	Glib::Threads::Mutex::Lock* source_lock = new Glib::Threads::Mutex::Lock (ms->mutex());
+	ms->invalidate(); // Release cached iterator's read lock on model
 	return WriteLock(new WriteLockImpl(source_lock, _lock, _control_lock));
 }
 
@@ -1853,6 +1854,10 @@ MidiModel::set_midi_source (boost::shared_ptr<MidiSource> s)
 {
 	boost::shared_ptr<MidiSource> old = _midi_source.lock ();
 
+	if (old) {
+		old->invalidate ();
+	}
+
 	_midi_source_connections.drop_connections ();
 
 	_midi_source = s;
diff --git a/libs/ardour/midi_region.cc b/libs/ardour/midi_region.cc
index cb0d103b1e..71fd796b81 100644
--- a/libs/ardour/midi_region.cc
+++ b/libs/ardour/midi_region.cc
@@ -421,6 +421,13 @@ MidiRegion::model_automation_state_changed (Evoral::Parameter const & p)
 	} else {
 		_filtered_parameters.insert (p);
 	}
+
+	/* the source will have an iterator into the model, and that iterator will have been set up
+	   for a given set of filtered_parameters, so now that we've changed that list we must invalidate
+	   the iterator.
+	*/
+	Glib::Threads::Mutex::Lock lm (midi_source(0)->mutex());
+	midi_source(0)->invalidate ();
 }
 
 /** This is called when a trim drag has resulted in a -ve _start time for this region.
diff --git a/libs/ardour/midi_source.cc b/libs/ardour/midi_source.cc
index 494bb513fe..823ca9dc5c 100644
--- a/libs/ardour/midi_source.cc
+++ b/libs/ardour/midi_source.cc
@@ -56,7 +56,9 @@ PBD::Signal1<void,MidiSource*> MidiSource::MidiSourceCreated;
 MidiSource::MidiSource (Session& s, string name, Source::Flag flags)
 	: Source(s, DataType::MIDI, name, flags)
 	, _writing(false)
+	, _model_iter_valid(false)
 	, _length_beats(0.0)
+	, _last_read_end(0)
 	, _capture_length(0)
 	, _capture_loop_length(0)
 {
@@ -65,7 +67,9 @@ MidiSource::MidiSource (Session& s, string name, Source::Flag flags)
 MidiSource::MidiSource (Session& s, const XMLNode& node)
 	: Source(s, node)
 	, _writing(false)
+	, _model_iter_valid(false)
 	, _length_beats(0.0)
+	, _last_read_end(0)
 	, _capture_length(0)
 	, _capture_loop_length(0)
 {
@@ -169,6 +173,13 @@ MidiSource::update_length (framecnt_t)
 	// You're not the boss of me!
 }
 
+void
+MidiSource::invalidate ()
+{
+	_model_iter_valid = false;
+	_model_iter.invalidate();
+}
+
 framecnt_t
 MidiSource::midi_read (Evoral::EventSink<framepos_t>&     dst,
                        framepos_t                         source_start,
@@ -186,11 +197,18 @@ MidiSource::midi_read (Evoral::EventSink<framepos_t>&     dst,
 	                             source_start, start, cnt, tracker, name()));
 
 	if (_model) {
-		// Read events up to end
-		const double start_beats = converter.from(start);
-		for (Evoral::Sequence<double>::const_iterator i = _model->begin(start_beats, false, filtered);
-		     i != _model->end();
-		     ++i) {
+		// Find appropriate model iterator
+		Evoral::Sequence<double>::const_iterator& i = _model_iter;
+		if (_last_read_end == 0 || start != _last_read_end || !_model_iter_valid) {
+			// Cached iterator is invalid, search for the first event past start
+			i                 = _model->begin(converter.from(start), false, filtered);
+			_model_iter_valid = true;
+		}
+
+		_last_read_end = start + cnt;
+
+		// Copy events in [start, start + cnt) into dst
+		for (; i != _model->end(); ++i) {
 			const framecnt_t time_frames = converter.to(i->time());
 			if (time_frames < start + cnt) {
 				// Offset by source start to convert event time to session time
@@ -225,7 +243,10 @@ MidiSource::midi_write (MidiRingBuffer<framepos_t>& source,
 
 	const framecnt_t ret = write_unlocked (source, source_start, cnt);
 
-	if (cnt != max_framecnt) {
+	if (cnt == max_framecnt) {
+		_last_read_end = 0;
+		invalidate();
+	} else {
 		_capture_length += cnt;
 	}
 
diff --git a/libs/ardour/smf_source.cc b/libs/ardour/smf_source.cc
index 3f22028e62..8a956495a5 100644
--- a/libs/ardour/smf_source.cc
+++ b/libs/ardour/smf_source.cc
@@ -701,6 +701,7 @@ SMFSource::load_model (bool lock, bool force_reload)
 
 	_model->end_write (Evoral::Sequence<Evoral::MusicalTime>::ResolveStuckNotes, _length_beats);
 	_model->set_edited (false);
+	invalidate();
 
 	free(buf);
 }
@@ -724,6 +725,8 @@ SMFSource::flush_midi ()
 	Evoral::SMF::end_write ();
 	/* data in the file means its no longer removable */
 	mark_nonremovable ();
+
+	invalidate();
 }
 
 void
diff --git a/libs/evoral/src/Sequence.cpp b/libs/evoral/src/Sequence.cpp
index 1cc8ff6e0f..082f8f24ea 100644
--- a/libs/evoral/src/Sequence.cpp
+++ b/libs/evoral/src/Sequence.cpp
@@ -61,10 +61,13 @@ namespace Evoral {
 template<typename Time>
 Sequence<Time>::const_iterator::const_iterator()
 	: _seq(NULL)
+	, _event(boost::shared_ptr< Event<Time> >(new Event<Time>()))
+	, _active_patch_change_message (0)
+	, _type(NIL)
 	, _is_end(true)
 	, _control_iter(_control_iters.end())
+	, _force_discrete(false)
 {
-	_event = boost::shared_ptr< Event<Time> >(new Event<Time>());
 }
 
 /** @param force_discrete true to force ControlLists to use discrete evaluation, otherwise false to get them to use their configured mode */
@@ -125,7 +128,7 @@ Sequence<Time>::const_iterator::const_iterator(const Sequence<Time>& seq, Time t
 		DEBUG_TRACE (DEBUG::Sequence, string_compose ("Iterator: control: %1\n", seq._type_map.to_symbol(i->first)));
 		double x, y;
 		bool ret;
-		if (_force_discrete) {
+		if (_force_discrete || i->second->list()->interpolation() == ControlList::Discrete) {
 			ret = i->second->list()->rt_safe_earliest_event_discrete_unlocked (t, x, y, true);
 		} else {
 			ret = i->second->list()->rt_safe_earliest_event_unlocked(t, x, y, true);
@@ -290,12 +293,10 @@ Sequence<Time>::const_iterator::operator++()
 		// Increment current controller iterator
 		if (_force_discrete || _control_iter->list->interpolation() == ControlList::Discrete) {
 			ret = _control_iter->list->rt_safe_earliest_event_discrete_unlocked (
-				_control_iter->x, x, y, false
-			                                                                     );
+				_control_iter->x, x, y, false);
 		} else {
 			ret = _control_iter->list->rt_safe_earliest_event_linear_unlocked (
-				_control_iter->x + time_between_interpolated_controller_outputs, x, y, false
-			                                                                   );
+				_control_iter->x + time_between_interpolated_controller_outputs, x, y, false);
 		}
 		assert(!ret || x > _control_iter->x);
 		if (ret) {