Domina 80E CH fault?

I can get hot water easily and constantly but the central heating doesnt work properly. The CH comes on when i switch it on but goes into standby mode after about 15mins, it then fires up again and will stay on 4mins and turns off again. The radiaters get hot but the boiler going off and back on every 5mins is ridiculous. If anyone can help give me ideas of what the problem could be i would be very grateful