The problem you might be having is possibly related to the PCV plate on the back of the manifold. At some time in the life of all the M-62 motors, they require it to be replaced. When they fail they show symptoms that you report. Go to Realoem. Com and look for part# 11-61-7-501-563. The plate can be removed from the car without pulling the manifold, but be careful not to strip the heads on the T-30 Torx screws or you will be removing the entire manifold. Also replace the throttle plate profile gasket as well. Has the valley pan ever been replaced?