My boiler makes banging noises when cutting out (Potterton PrimaB)?

When the boiler cuts out it makes enormous banging noises. The pilot usually remains alight and the boiler will again restart without any interference. I have replaced the pump but no improvement. Sometimes the boiler will go out and sometimes the cut-out switch will need pressing. The boiler is a Potterton PrimaB, balanced flue system. The system is microbore which is showing some signs of being gummed up, even after flushing through. Any ideas welcome.