puzzling problem with baxi inset 3 back boiler?

Boiler continually locks out. After reset would run for a several hours or more before locking out again. Following troubleshooting guide in baxi manual decided to replace pcb. since then boiler runs for 2 minutes then locks out for 10, does not reset, goes directly to red then goes to green when its ready no input from me, manual says it may be potentiometer. anybody out there with the answer