Pressure increase and decrease problem?

I have a vaillant combi turbo 824e and have noticed the pressure dropping to well below 1 bar. I top up and bleed the system . The system when cheating on goes up over to 3bar and relief kicks in. After central heating off the pressure drops over about 2hrs to 0.6 or 0.4 bar. What is wrong? I've been told it is probably the expansion vessel which is very costly to replace and I should fit one on the external pipe somewhere suitable. Any advice