hello mate usually the dump valve doesnt go off unless your revving the nuts off of it when its stationary if thats whats happening then thats nothing to worry about but on the samehand it could be your dump valve causing the problems,try taking it off and blocking the pipes connected to it and see if that makes a difference and if your dump valve is a diaphram type then they are no good for that car anyway you need a twin piston type. it sounds like you have a vacuum leak somewhere try checking all your pipes to your intake,intercooler,brake servo and all your breather pipes also check to make sure your dipstick is seated properly and your oil filler cap is sealed properly as all of these can cause the problems you are describing.