Hi.. My heating keeps shutting down and staying off for several hours. Then it will kick back up again and stay hot for several hours. I've just had a new pump put in and have flushed the system several times (I was told I may have sludge in the system causing it). Flushing the system has not improved it at all. The hot water is not affected, only the heating. When it comes on it gets really hot and but will stay on for ages before it shuts off again. Any advise would be appreciated. thanks..