I may be stating the bleedin' obvious, but have you checked that the radiators are all properly bled? I had similar problems with boiler pressure all over the shop, which baffled me and a British Gas engineer - until it occurred to us to check that. When you're deeply immersed in technicalities of boiler pressure shooting up and down, release valves operating, etc, it's easy to forget something simple. My problem started with a leaking expansion vessel, but the bizarre pressure problems came afterwards. If there is air in the system, it will expand at a much greater rate than the water when heated up, so it fires the pressure relief valves, and so much water is ejected that the boiler looks (and is) unpressurised when it cools down again. So, it then looks as though fixing the expansion vessel hasn't cured anything. Sorry if I'm insulting your intelligence, but it did happen to me (and to that BG engineer!).