Gloworm fuelsaver f boiler?

Hello my gloworm fuelsaver f seems to keep firing up every 2 minutes ? from cold the boiler will run for a lot longer until it seems it reaches a set temp then it starts the cycle of firing every 2 minutes and runs for a 1 minute it makes no difference if i turn up the room stat or have the boiler stat up to max ? apart from that the heating works fine and the hot water works fine , apart from the cycle of the firing up all pumps and motoriszed valves working fine . regards dean