puma 80 diverter valve?

hi
I have got a puma potterton 80 which is turning into a nightmare 8 months ago the heating wasnt coming on as it should have been but hot water was working fine the problem was diagnosed as being due to the wax capsule in the diverter valve sticking in the hot water position a new diverter valve was fitted as well as new pump and thermister (h/w & c/h) the boiler is now showing similar sympoms the central heating doesnt come on at all and sporadically the pilot light goes out coupled with loud banging noise and steam overheating i think but this happens even when not being used.My gas fitter who is a good friend is a bit stumped as numerous checks on all components suggest it needs a new diverter valve again wondered if anybody had experienced similar probs and can think of anything that we have overlooked any help would be great thanks

Rob