My Vaillant VCW GB 221 H boiler will not produce enough heat. Well, not continously anyway. It gets up to temperature OK fairly quickly but then cuts the gas off not turning it back on until the water temperature has dropped to around 40 degrees. It will then go back up to 90 degrees or whatever I have set it to and cut off again repeating this cycle continuosly. This means that the average water temperature is only luke warm and the radiators never get the chance to properly heat up. The hot water from the taps is affected in the same way. So I can never get a nice hot bath, only luke warm. I am wondering if there is something that needs adjusting or is there a fault I can fix? It has been like it from new a few years ago, I have just decided I am fed up with it and am wondering if it was ever installed properly in the first place?