Possibly too much spring pressure? A few years back I had the unfortunate luck of wiping out four or five cams in one season. The spring pressures were supposedly good, lifters turned nicely in their bores, engine always started easily, used proper break in procedures.......blah, blah, blah. The ONLY thing I never verified was my spring pressures. I trusted the builder. Finally scrapped those heads and used an untouched set of X heads, no problems since. I killed two Purple Shafts, and three Comps. Same lifters every time.........