pump overrun?

Vaillant 221 H combi,The pump overruns for a long time after the burner has switched off after reaching temp and burner wont fire up again.The only way to get things going again with the burner igniting is to switch C/H off for about 15 mins,it some times but not as often does it on the domestic side.I've changed the two PCBs in the silver box at the front to no avail.What else could it be as Ive had a gas fitter look at it and he said Oh thats normal(dick ed).Hope some one can put me on the right track CHEERs in anticipation