Burner on Sime Friendly Format 80E switches off?

Hi and help.
My friendly format 80E is not so friendly, The burner in central heating mode will just switch of the gas to the burner, but all the other items, ie fan and pump still run.This will happen 6 to 7 times in the first hour, then how it will run fine.
Hot water fine, Parts that have been changed are both the PCB, divertor valve had kit fitted, pressure safety valve and also the fan.
Only gas valve left and have a new unit?
What could be the issue please?