flashing reset and faster flashing burner leds?

sometimes starts ok and goes faulty after a minute, sometimes 30 minutes, sometimes ok all day. To get out of fault condition turning of water temp rotary control on and off several times on boiler sometimes/usually works and works ok for rest of time, sometimes not. If left in flashing state it sometimes sorts its self out and continues to work ok for rest of time set,sometimes not. Also swithing on/off mains supply can resolve it in conjunction with temp rotary control off, until a steady reset led is achieved, then turning up rotary heat control on boiler fires it up and again can be ok for rest of time sometimes not. Has had 2 new pcbs in last 6yrs last one (which looks slightly differrent) was 3yrs ago. boiler is 10yrs old. doesnt seem to be any pattern to fault although when colder outside seems more problamatic. kind regards