What is my problem, Worcester Bosch Grenstar25si/30si?

The pressure has become unstable on my combi greenstar. Just about holds up when heating on. If I isolate the heating and run the hot water tap the pressure drops significantly. There is no water coming from the pressure release valve out side. My Plumber is faurly sure it is not a leak on the heating system. Could it be the expansion Vessel. The boiler is only two years old.