Boat is 21 ft Eliminator SC, Dominator AA pump, Engine is 460 BBF twin turbo 8:25:1 CR, DV3E heads, mildly ported, std valves, big PFM IC, Draw thru 850 Holley (1050 proform body), run 91 octane in primaries, 110 in secondaries. Checked A/F with LM-1 system and had solid 12.0:1 in higher RPM ranges. Ran it fairly hard in morning (a couple of WOT 30 second runs at 5400 RPM, 10 psi boost). Did blow one of the compressor to IC hoses, but reconnected it and it ran fine. A couple hours later I took the boys on a 15 minute sustained 4000 RPM tube ride. Then after about two hours of listening music on the beach, I idled it around between 800 and 1500 to charge the battery and it ran fine. Then about two hours later I started it up to put it back on the trailer and it runs like it's on 7 cylinders. I get it on the trailer and pull the plugs. They look fine. I run a compression check and get 120 in the first 7 and nothing in 8. I get the exhaust off tonight and can see the seat on top of the valve. OK, anyone have ideas as to why this happened and how to minimize the possibility of it happening again? Would tired valve springs or lifter issues cause this? I've been running this set-up for years w/o major problems, so it was kind of a drag loosing a couple days on the water.