diff --git a/Machines/Atari2600/Atari2600.cpp b/Machines/Atari2600/Atari2600.cpp
index a5113400f..429a084dc 100644
--- a/Machines/Atari2600/Atari2600.cpp
+++ b/Machines/Atari2600/Atari2600.cpp
@@ -30,18 +30,37 @@ Machine::Machine() :
 void Machine::setup_output(float aspect_ratio)
 {
 	_crt = new Outputs::CRT::CRT(228, 1, 263, Outputs::CRT::ColourSpace::YIQ, 228, 1, 1);
+
+	// this is the NTSC phase offset function; see below for PAL
 	_crt->set_composite_sampling_function(
 		"float composite_sample(usampler2D texID, vec2 coordinate, vec2 iCoordinate, float phase, float amplitude)\n"
 		"{"
 			"uint c = texture(texID, coordinate).r;"
 			"uint y = (c >> 1) & 7u;"
 			"uint iPhase = (c >> 4);"
-			"float phaseOffset = 6.283185308 * float(iPhase + 13u) / 16.0;"
+
+			"float phaseOffset = 6.283185308 * float(iPhase + 13u) / 14.0;"
 			"return (float(y) / 7.0) * (1.0 - amplitude) + step(1, iPhase) * amplitude * cos(phase + phaseOffset);"
 		"}");
 	_crt->set_output_device(Outputs::CRT::Television);
 }
 
+void Machine::switch_region()
+{
+	// the PAL function
+	_crt->set_composite_sampling_function(
+		"float composite_sample(usampler2D texID, vec2 coordinate, vec2 iCoordinate, float phase, float amplitude)\n"
+		"{"
+			"uint c = texture(texID, coordinate).r;"
+			"uint y = (c >> 1) & 7u;"
+			"uint iPhase = (c >> 4);"
+
+			"float phaseOffset = (0.5 + 2.0 * (float(iPhase&1u) - 0.5) * (float((iPhase >> 1) + (iPhase&1u)) / 14.0));"
+			"return (float(y) / 7.0) * (1.0 - amplitude) + step(2, iPhase) * step(iPhase, 13) * amplitude * cos(phase + 6.283185308 * phaseOffset);"
+		"}");
+	_crt->set_new_timing(228, 312, Outputs::CRT::ColourSpace::YUV, 228, 1);
+}
+
 void Machine::close_output()
 {
 	delete _crt;
@@ -54,11 +73,6 @@ Machine::~Machine()
 	close_output();
 }
 
-void Machine::switch_region()
-{
-	_crt->set_new_timing(228, 312, Outputs::CRT::ColourSpace::YUV, 228, 1);
-}
-
 void Machine::get_output_pixel(uint8_t *pixel, int offset)
 {
 	// get the playfield pixel and hence a proposed colour
diff --git a/Outputs/CRT/CRT.cpp b/Outputs/CRT/CRT.cpp
index 92f2c7a96..e1fe9ca0c 100644
--- a/Outputs/CRT/CRT.cpp
+++ b/Outputs/CRT/CRT.cpp
@@ -216,9 +216,13 @@ void CRT::advance_cycles(unsigned int number_of_cycles, unsigned int source_divi
 			if(_delegate)
 			{
 				_frames_since_last_delegate_call++;
-				if(_frames_since_last_delegate_call == 100)
+				if(_frames_since_last_delegate_call == 20)
 				{
+					// Yuck: to deal with the permitted ability of the delegate to make CRT changes that require the lock to be
+					// asserted during the delegate call, temporarily release the lock. TODO: find a less blunt instrument.
+					_openGL_output_builder->unlock_output();
 					_delegate->crt_did_end_batch_of_frames(this, _frames_since_last_delegate_call, _vertical_flywheel->get_and_reset_number_of_surprises());
+					_openGL_output_builder->lock_output();
 					_frames_since_last_delegate_call = 0;
 				}
 			}
diff --git a/Outputs/CRT/Internals/CRTOpenGL.cpp b/Outputs/CRT/Internals/CRTOpenGL.cpp
index ecd1eb6f4..f929da5b2 100644
--- a/Outputs/CRT/Internals/CRTOpenGL.cpp
+++ b/Outputs/CRT/Internals/CRTOpenGL.cpp
@@ -337,7 +337,11 @@ void OpenGLOutputBuilder::set_openGL_context_will_change(bool should_delete_reso
 
 void OpenGLOutputBuilder::set_composite_sampling_function(const char *shader)
 {
+	_output_mutex->lock();
 	_composite_shader = strdup(shader);
+	output_shader_program = nullptr;
+	framebuffer = nullptr;
+	_output_mutex->unlock();
 }
 
 void OpenGLOutputBuilder::set_rgb_sampling_function(const char *shader)