how can i fix my baxi combi 80 eco?

My Boiler keeps firing on hot water when no hot water is been drawen. The hot water DHW Pressure
Differential Valve does not operate ( the pin has not operated the micro swith). I can some times get the boiler fire just on central heating so when I remove the wire loop from the wiring terminal the burner stops but some times the boiler is operating and the radiators get much warmer than they should you remove the wiring loop and the burner continues to fire (at higher demand as if heating hot water). I wonder if it could be a problem with the PCB. The Safety Limit Thermostat does not knock the boiler off when the burner is operating for hot water. The problem is intermittent.