no hot water baxi combi 105e?

Hi. I'm looking for some help with a baxi 105e combi boiler. Central heating works fine but hot water only just runs warm and only if you have the tap open a small amount. When running a bath(very slowly) The burner light is on for 30 or 40 seconds then cuts out for about 20 seconds. Having the preheat on or off makes no dirrerence. Fortunatley I'm on homecare with British gas and they have been out probably half a dozen times with no sucess and have replaced various sensors the pump, heat exchanger, I think, the diverter valve. They have also fitted an external expansion chamber and a magiclean but I think that was to do with a problem with the central heating. Getting really frustated now!!!
ps also had new pcb