Potterton Profile 50e - keeps resetting?

My profile 50e resets itself at least once/twice a day which means I can't rely on it to heat water first thing in the morning or turn the heating on when I return from work. The motorised valve is a year old, the pump is new, the overheat thermostat has been replaced - nothing to seems to resolve this frustrating intermittent fault. The bolier is about 9 years old but works perfectly well when it stays on. The flue isn't blocked and the boiler has recently been serviced as well. Any suggestions would be greatly appreciated, thanks.