diff --git a/include/AudioJack.h b/include/AudioJack.h
index 9bbb3bd48..60adfc9a5 100644
--- a/include/AudioJack.h
+++ b/include/AudioJack.h
@@ -37,6 +37,7 @@
 #include <QtCore/QVector>
 #include <QtCore/QList>
 #include <QtCore/QMap>
+#include <QMutexLocker>
 
 #include "AudioDevice.h"
 #include "AudioDeviceSetupWidget.h"
@@ -107,6 +108,7 @@ private:
 
 	bool m_active;
 	bool m_stopped;
+	QMutex m_processingMutex;
 
 	MidiJack *m_midiClient;
 	QVector<jack_port_t *> m_outputPorts;
diff --git a/src/core/audio/AudioJack.cpp b/src/core/audio/AudioJack.cpp
index 24f7b2f4f..4d730eed4 100644
--- a/src/core/audio/AudioJack.cpp
+++ b/src/core/audio/AudioJack.cpp
@@ -71,6 +71,7 @@ AudioJack::AudioJack( bool & _success_ful, Mixer*  _mixer ) :
 
 AudioJack::~AudioJack()
 {
+	stopProcessing();
 #ifdef AUDIO_PORT_SUPPORT
 	while( m_portMap.size() )
 	{
@@ -200,6 +201,7 @@ bool AudioJack::initJackClient()
 
 void AudioJack::startProcessing()
 {
+	QMutexLocker m( &m_processingMutex );
 	m_stopped = false;
 
 	if( m_active || m_client == NULL )
@@ -252,6 +254,8 @@ void AudioJack::startProcessing()
 
 void AudioJack::stopProcessing()
 {
+	QMutexLocker m( &m_processingMutex );
+	m_stopped = true;
 }
 
 
@@ -338,6 +342,8 @@ void AudioJack::renamePort( AudioPort * _port )
 
 int AudioJack::processCallback( jack_nframes_t _nframes, void * _udata )
 {
+	QMutexLocker m( &m_processingMutex );
+
 	// do midi processing first so that midi input can
 	// add to the following sound processing
 	if( m_midiClient && _nframes > 0 )
@@ -397,15 +403,15 @@ int AudioJack::processCallback( jack_nframes_t _nframes, void * _udata )
 		if( m_framesDoneInCurBuf == m_framesToDoInCurBuf )
 		{
 			m_framesToDoInCurBuf = getNextBuffer( m_outBuf );
+			m_framesDoneInCurBuf = 0;
 			if( !m_framesToDoInCurBuf )
 			{
-				m_stopped = true;
+				break;
 			}
-			m_framesDoneInCurBuf = 0;
 		}
 	}
 
-	if( m_stopped == true )
+	if( _nframes != done )
 	{
 		for( int c = 0; c < channels(); ++c )
 		{