hot water is running warm/cold?

I have a potterton performa 28i combination boiler less than 5 yrs old. Worked fine until few months ago. Called in plumber as boiler was losing pressure and hot water running hot/cold. We have had the safety valve, expansion vessel, domestic hot water heat exchanger, pcb board and micro switches ALL replaced at great expense. Pressure now OK hot water problem is worse than ever - burner kicks in but only lasts about 20 seconds then goes out, if lucky we can get lukewarm water before goes cold again. AT WITS END!!