How to fix my old 350 Worcester combo boiler?

Hi my boiler keeps locking out with the central heating on. It seems fine when it's heating hot water. I noticed lastnight that the radiators got excessively hot and the system pressure peaked at just over 3bar shortly before it locked out. (usually around 2bar with central heating on whenever I've looked before).
If I use the reset on top of the boiler it works fine again for a day or so. Any ideas or known common faults? Any info will be greatly recieved.