Combi boiler faults?

I have a Wickes Combi Boiler 102. it has started to keep continuously stopping and starting whilst heating the radiators. as soon as the Hot tap is turned on the boiler goes to lockout. The pump is working ok, the pressure is continuous at 1 and half. it has been looked at twice by heating specialists. no real ideas ? This boiler is only just 4 years old.