I think found the problem (and I should have done this a LONG time ago).
Rented a compression tester from AutoZone.
Pulled the plugs - 1, 2, 3 and 4 look fine. 5 and 6 are not so good.....the plugs are completely black.
Anyway, ran the compression test, starting with cylinders 2, 4 and 6 - they are on the front of the engine and easiest to get to.
Cylinder 2 - 175 PSI
Cylinder 4 - 170 PSI
Cylinder 6 - 15 PSI (!!!!) - I stopped after I saw this one.
Yessiree, Houston, we have a problem. I don't hear any banging when turning the motor over, so I don't think I dropped a valve and nuked the piston. I'm thinking either bad rings or a blown head gasket.
So at best, I was only running on 5 cylinders. But given the state of the #5 plug, I wouldn't be suprised if I was only running on 4. Dammit!!!
Pics of the plugs (5 and 6 are carbon-fouled):
Motor comes back out next weekend.