potterton prima f fault?

I have a prima f boiler which was working fine untill about 3-4 weeks ago. The problem is the the boiler will fire up ok and work alright but suddenly it will not work when the thermosat kicks in. It doesnt happen all the time but about once a day. The only way I can get it to work again is to switch the control / timer panel off at the mains and then switch it back on. It will then fire up again. Does anyone know what could be the problem? Thanks