Boiler cutting out?

We had a new radiator fitted, all heated up fine. 2 days later the boiler started cutting out. Plumber came back and bled the system, issue returned.

Fitted a vent on boiler, heated fine, 1/2 days later boiler cut out again.

British Gas came out and said we have sludge and need to replace H pipes. All replaced. Issue returned. Power flush done. Issue returned.

Heating works fine until the new radiator is turned on.

Running out of ideas now!! Help pleaseee