Vokera Compact 28 turning on and off?

Hi I have a problem with a Vokera Compact 28

When heat is demanded, CH or DHW the boiler goes through its cycle ( green LED flashing fast ) and ignites ( green LED steady ) ..... all fine up to now.
Then the boiler tuns off as if a thermistor had switched, it then cycles through ignition again until green LED steady then turns off again. It sits there happily doing this for as long as there is a heat demand........ I have changed PCB and thermistors.........Any ideas ?

Cheers