Lukewarm radiators and fluctuating water temperature?

I have a Baxi 105e combi boiler. The problem started about 2 months ago. The boiler fires up as normal, but the radiators only get luke warm. Some are cold. The boiler seems to cool down before it fires up again. At the same time, the hot water fluctuates between hot and nearly cold. I have to keep walking in and out of the shower, catching it when the temp is ok. Any ideas gratefully appreciated