RAVENHEAT LS80 in consistent DHW + pressure probs?

I have a ravenheat LS80T and when i don't use the Heating the pressure stays perfect but when I use the heating and the system goes cold after heating has switched of the pressure drops to zero thus stopping the boiler all together, I have changed the PRV but the pressure still drops and there are no leaks anywhere. all so the hot water temperature won't stabalise goes really hot then cuts out making it cold then cuts back in again, I have changed the DHW/CH potensiomiters which made no difference at all.

si689
November 2008