Did he have any seat/guide work done on the head? If he had seats installed, did the shop check the valve stem height before and after seat install? I've seen head work fail because valve stem height was not checked. Also if new guides were installed, is the guide height the same as the previous guides. How about valve springs, were they replaced, and are they possibly different than original, and bottoming out? Just a few ideas off the top of my head. Take it for what it's worth.
