Hi Stu. Lots of good advice here. But with all this advice, it seems your chasing your tail, "fixing" without a definite direction. Problem is, your car is at your house, not here. Go to where you can do these tests on the engine, with someone good at this sort of thing. I just got done going through something like this too. But I finally just went through all the tests, and found the problem, fixed it. This sounds like an older motor you've got. Take the valve covers off, pull the plugs, spine the motor. Do a look-see at the valve train. Do the comp. test, a leak down, and test the spring pressure (look for broken springs, and bent push rods). This will rule out a few things, or point you in the right direction. I know I'm just covering some of the same things others have told you, but you need to do these things!
Take a look at these you tubes, just a base line for ya.
Mike
PS, one other thing, put that monster on a scope, see what's up with the spark.
http://www.youtube.com/watch?v=M92CGjpiMs4http://www.youtube.com/watch?v=nJ7RXu4nxkc
