baxi solo 2 50pf only fires up occassionally?

I have a baxi solo 2 50pf which only fires up occassionally. The "boiler on" light comes on but nothing else and there is only intermittent power to the fan. The following parts have been replaced as brand new:

fan - pcb - thermostat sensor

somebody please help as every time i replace something it appears to work but then boiler shuts back off and doesn't come back on.