baxi boiler?

we have a baxi combi instant boiler which keeps turning itself off,can be on for 20 minutes or several hours but it always flicks itself off and the light flashes ,real pain as we cant set the timer when we are at work as it wont heat up.if i turn to reset or run the hot water it comes on again,pressure seems to drop on the system and when the boiler is on the pressure goes very high,any ideas anyone?