I have an Apollo boiler for central heating and hot water and must be about 14 years old. Most of the time it works fine and will run for hours with no problems. But now and then the pump seems to stop even though the burner is still going, which causes boiler to start banging away. The problem seems to be aggravated if the hot water has to be heated at the same time. This may be a wild guess, but I think the pump stops whilst the valve switches from the hot water back to the central heating, but never starts back up.
PS – My neighbour has exactly the same problem too. Any ideas?