problem with protherm 80e?

i have a protherm 80e combi boiler. the hot water is fine but the central heating is not working properly. when you switch the heating on the boiler fires up, reaches the set temperature and the pump kicks in then the boiler cuts off, the pump still runs but the rads dont get hot. the temperature seems to take a long time to drop down to the point where the boiler fires up again. is this to do with the 3 way valve or the microswitch