worcester bosch 28cdi DHW problems?

Hi, having had at least a couple of years good use of this boiler,recently the hot water when run for 2 or 3 minutes, goes cold. the boiler cuts out and then within a few seconds relights and heats the water for 2 or 3 minutes then cuts out again ( very annoying when in the shower and impossible to have a warm bath !). i have checked the diverter valve which hits the microswitch and stays there until the tap is turned off. This occurs however hot you set the water temperature. Anyone got any ideas what the problem could be ?