Ariston Eurocombi (A/23 MFFI) switches off after 45 seconds?

My Ariston Eurocombi (A/23 MFFI) switches off after 45 seconds of heating, then comes back on a minute later. It repeats this cycle endlessly.
The hot water is very hot when the heating is on, even though set at lowest setting.
If I turn the heating off the radiators cool (from bottom to top) to room temp in 15 minutes.
When I use hot water in the bathroom the pipes bang noisily.
I have had the same engineer from the preferred company of my insurer visit twice. He's replaced the temperature probe and a couple of circuit boards (C MI/FFI & EI A FFI/P ASIC), but to no avail.
Has any one got any suggestions as to what might be causing the problem?
Thanks

Susan
December 2011