Vokera Compact 24 Fault (Red light)?

My boiler keeps tripping to internal fault (red light) after it has been on for a hour or so. It only appears to do this when in auto mode and set up with the clock?. Once reset the boiler works ok water and heating. I have set the position to auto and it does not seem to go on to the internal fault mode. Pressure is ok etc. Any ideas would be appreciated.