system only fires up for 10 seconds then cuts out then cut back in after a minute or so the baxi boiler is not a sealed system it has pump and 2 motorized valves (i take it this makes it a combi boiler) and a tank in the loft i have changed pump and valves tank in the loft is fine but the burner still only fires up for a short time ...getting desperate ..is there a thermostat in the boiler its self that could be faulty?