Baxi Eco 80 keeps tripping safetly limit thermostat?

Hi, been reseting my 80 Eco infrequently for some time now. It used to trip when the dhw was used. Had a baxi engineer out and he wanted to change the PCB to make the pump run on. After bleeding the radiaters on a regular basis for a while, this problem went away. Now it trips when the burner goes out once thermostat temp is reached (on central heating side). Any ideas what could cause the problem? Should the pump always run on for a short while once the burners gone out? Any help at all would be appreciated. Cheers