How can I fix my Vokera Maxin 28e boiler?

My Vokera Maxin 28e combi boiler continues working normally for 30mins sometimes up to 2 hours before shutting down. When going it works normally heating the radiators and providing hot water. However after a period of time it shuts down completely and the red lock out reset button comes on. When pressing the reset button it goes off and the boiler again works normally for up to 2 hours. Anyone any ideas?