poterton performer 24?

my c/h is working fine but when the hot water is run it is only luke warm then it goes cold this is because the boiler fires up for 10 seconds then goes out for 30 seconds then fires up again .the only way to get some hot water is to run the hot tap in the bath and the cold tap in the sink on full.ihave replaced the complete diverter valve,pcb board,and the thermostat sensor but still no hot water

tim
February 2009