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hot water problem Glowworm 80 P?

Glowworm 80 P boiler has an internittent hot water problem.
The hot water will work for a while then the burner shuts down and re starts a few seconds later.
we had a burst water main before the problem started and a lot of debris came through when the pipe was reconnected
What could be causing this?
ed, October 2013
blocked heat exchanger

A1, October 2013