Problem with hot water demand?

I have a vaillant 824 E, problem with hot water demand - boiler kicks in on start up but temperature on boiler races up to 100 + and so stops boiler from firing when over 80 deg (ie cuts out) also when temperature drops below 70 deg the boiler doesn't always kick back in, when divertor operates their appears to be abuild up of pressure in the system which sounds like the pump momentarily run dry Heating system run ok but can't deliver any hot water for aperiod- divertor valve been replaced which stopped a series of fault codes from displaying and stopping boiler but don't appear tobe able to maintain a constant hot water demand ! Any suggestions...