No one has taken my suggestion that it may be the lifters themselves. I may be wrong but I still believe the culprit may lie there. Perhaps the wrong lifters, perhaps worn out, maybe something else but it sounds suspicious to me and is certainly worth a look and not too difficult to do. The one hole I see in my logic is the fact that it has always been intakes that have broken (?) If I'm all wet it sure won't be the first time.
