Micron 40ff heating up then cooling right down before starting up again. ?

Please help I have a Micron 40ff with radiators with TRV's & a room stat, I replaced the old style PCB about 3 years ago for the 2000801990 model after the usual burnt board.
The fault I have now is the heating comes on & heats the radiators up perfectly to piping hot, the burner then turns off I would have thought due to the thermistor detecting the water in the heat exchanger is at max 82 deg C, the pump now continues to run & the radiators start to cool down at which point the burner if I'm correct should re-start, It actually does not start until the radiators are little more then warm.
I've checked the resistance of the thermistor at cold 12.5kohms, the PCB has no signs of dry joints or burn marks. The boiler always ignites first time when it finally decides to start up, If I turn the boiler off either with the temperature knob or the programmer it will fire up again & everything's fine until it turns off when the radiators are hot, It does exactly the same if its set to hot water or both, any Ideas