worcester 28i playing up?

the fault light comes on and the fan/blower keeps going i have done every thing you sugested and it does work for a day or two ,what i have to do now is switch the power off wait a few seconds switch it back on and give it a tap on the bottom left side of the boiler and i mean a tap and it clicks a few times and it egnites and runs great for a few hours and some times days.i have looked for a loose conection to know avail the place were i tap is were the heater waterbutton from summer hot water to full heating is ,i need help please