Was going to suggest a compression test; PB is right though, check the oil for emulsification, a sure sign of coolant ingress, as I'm sure you know. The plug blowing out on start-up could be caused by the position of the piston when the engine is shut down, especially if it doesn't happen every time. Let's hope it's only worn threads. But, check that coolant leak; have you tried adding 'Radweld' (don't know what it's called in the 'States) or a raw egg to the radiator? Nearly always works!
Terry.
PS, just remembered, on the V8 fitted to Land Rovers/Range Rovers (basically the original GM design), similar faults could occur after hard off-road use, like trialling, expeditions, competitions etc. Seem to recall it was something to do with the breather being blocked, and / or the oil filter, sometimes the oil pump also, being dirty/blocked. Just a thought!