Hi guys hoping someone can help

I have a netaheat potterton 10-16 electronic and the boiler when cold fires up and can run for 7 hours no problem heating the house well and then it will stop heating the house, the pilot light remain on and there is park but main burner wont light

If I leave the boiler to cool right down it will then run fine, for example if I wait till the next day it starts first time and then fter 5-7 hours its turns off.

I have tried a new pcb and it did the exact same thing with the new pcb

It works perfectly for that 5-7 hours but then just wont light the main burner until fully cold.

If anyone can tell me what this is likely to be I would be most grateful

thanks in advance for any help given.

Dev, October 2016