Jeff, here's my $0.02.
Motor has had some work done, piston window has been opened up, there is copper coat on the gasket, if there is a single head gasket more then likely the cylinders have been lifted. measure the base gasket thickness and let us know. Take a picture of the cylinder from different angles and post. Make sure you show one from the intake side as well. The problem is definatley like 3:16 stated is detonation but why. 90 % of the time I'll agree that bad fuel will be the culprit. Other things to check are the rubber intake manifolds if the are sucking air it will cause a lean condition in that cylinder only. Also if you can measure the squish that would be useful in determining how much work has been done to the motor.
/Dan