Glow worm 30ci water temperatue fluctuating?

Hi, problem with my glow worm 30ci boiler:the water temperature fluctuates from hot to cold ,back to hot to cold etc. When I open a hot water tap, I can hear the burners ignite but the water only gets luke warm, then I hear more burners ignite and the water gets hot but only for a few seconds before returning to only luke warm.The boiler then ignites again to give me a few more seconds of hot water before returning to luke warm, then ignites again to give me a few more seconds of hot water and so on. The boiler doesn't switch off while the water is flowing , so I presume there are 2 sets of burners. I had this problem fixed under guarantee once before. The repairer hadn't got a clue as to the fault and replaced almost every part in the boiler before the fault was rectified and he didn't know which part cured the fault. Thus, I don't want to go down this road again, now that it is out of warrranty. The boiler is 4 years old. Any ideas