Potterton Puma 80E Combi overheats and trips out on hotwater cycle?

When the hot water is adjusted or moved from its minimum setting the boiler trips out. Why?
Almost every part on this boiler has been replaced with new parts; Gas valves, diverter valve, wax stat etc.,
All sensors/stats have been changed. New boards etc.,
The only things not yet changed are the heat exchnager and the expansion tank. (original burner still in use and in good condition.)
When the boiler is running you can hear a sound not unlike a boiling kettle. If the water is getting too hot, is the heat exchanger scaled up and would that cause the boiler to trip out on the hot water cycle?
Central heating runs with no problems.
Replacement pump fitted three years ago.
Spent too much money on this boiler to scrap it. Can this problem be fixed?