worcester 240 rsf pressure rise then shut off.......help!!

[helm]

Hi all , i have a worcester 240 rsf combi. i recently posted on here about a pressure drop problem and since then have sorted that one by changing the prv, now i have a new problem, whenever i draw from the hot water or put the central heating on the pressure shoots up and the system shuts down(the flame goes out)..i checked out the pressure vessel and found water coming out from the air valve( a ruptured diaphram)..so i've changed the pressure vessel and now the system raises and shuts down when i start either the hot water or the heating. Prior to shuting down there is a noise either coming from the valve that switches from hot water to central heating or the pump.It sounds like a whirring or screetching noise and continues making the noise even after the system has shut down.
A plumber has suggested it sounds like a pressure problem still,but after replacing the pressure vessel and the prv i cant see where.......

any help would be very much appreciated guys ,

thanks in advance