I have a Potterton profile balanced flue gas fired boiler. It is now behaving a bit strangely. The heating and hot water are OK but it will go on for about 2 seconds then off 2 secs and repeat this for 30 or 40 times and only then does it go on for a normal period of time e.g. 20 secs and upwards. Is this unit on the blink? I have checked the small water tank that supplies the central heating and that is OK. I have checked for any air locks in the CH but that's OK.