some info.....preignition can result from hot spots in the combustion chamber, over-advanced ignition timing, lean air/fuel mixtures, defective distributor advance mechanisms, deposits in the combustion chamber, defective cylinderhead or manifold gaskets. To avoid pre-ignition, a thorough tune-up is required before installing new spark plugs.
start with a compression test and work your way back...id keep in mind if thats melted several plugs im sure there is debris floating around in there that could be gouging out the cylinder walls, messing with the piston, rings, valves. has it been knocking at all?