you are experiencing a valve problem. they are recessed into the head and when the lifters get "pumped up" the valves stay open. when you shut down they pressure drops and process starts over. to confirm you can shim the rocker arm, don't block the oil passage. one of the symptoms that may be present that will con firm the valve issue would be the loss of compression sound when you immediately try to restart at time of failure. this was a common problem with some six series units. i think the consensus was excessive valve spring strength.