Driving my Bee home from a show a couple weeks ago, I noticed a tapping in the engine. I tore it open this weekend and the #8 exhaust lifter is concaved and looks like the lobe is shot. The engine is a 383 with MP 509 cam that was built 23 years ago and has about 10k on it. I just put a set of MP aluminum 452's on it a couple months ago (300 miles), stock rockers with correct length push rods. I am going to pull it clean it out and put another 509 in it for now, until my 489 stroker is done. I do not want it to happen again, What possible causes should I look for? Thanks.