Ditto on the timing. See if the distributor is loose, which can happen depending on the situation. If previously removed and re-installed, the distributor can sometimes bind and not go all the way down when meshing with the camshaft. After tightening the holding clamp the the distributor can later on drop in place and the clamp is now loose. When I re-install a distributor, I like to rotate the shaft back and forth via the rotor (jiggle it).
