what is wrong with boiler?

I ahve a Potterton combi boiler and 3weeks ago it started firing for 3 mins then turning off for 2 mins then on for 3 mins and off for 2 and so on. its always playing catch up! The radiators are hot but heat can't be felt unless you're next to the radiators. The room is warm but never comfortably so. Is it the boiler? If so what could be wrong?