My Potterton Puma 80 has pressure build up problem!?

When the boiler is providing hot water only everything is fine, operating without any problem. But when the central heating is turned on, the radiators get hot and the water is circulating, and, but the boiler pressure goes up from 1 1/2 bar to 4, it just keeps increasing! Thus intern releasing the pressure relief valve set at 3 bar, and then when everything cools down, the low pressure light comes on. The boiler gets topped up and the whole circle starts again. Any ideas as to what might be causing this???