PROTHERM 80E - Boiler cuts out ???

Boiler will run DHW / CH and for no apparent reason cut out, particularly annoying half way through a shower.

Sometimes shows F1 error code, sometimes doesn't.
Have to turn it on and off again at the mains switch to get it to work, occasionally leaving it 10 mins or so.

Had overheat thermistor changed, still the same.

It seems these Protherm boilers have a few common faults, although I haven't seen any specific answers to this problem - would be most grateful for any guidancel (having already spent a fortune with no luck)