Vaillant vcw sine T3W pressure drop?

Hi

I have a vaillant vcw sine t3w boiler which is fine on the water circuit, but when central heating side cuts in the pressure increases to around 3+ bar but then after boiler cuts out the pressure drops to which point it overheats and then wont fire up again. I am keeping my eye on it and will have to top it up daily during winter to stop it from cutting out. During the summer when central heating is not used it is fine and doesnt lose pressure. Any suggestions on possible causes/fixes would be appreciated.