Mike,
It is sounding more and more like you simply don't have enough radiator. One quick and dirty way to find out is to leave the cowling off the plane and take a flight. I'll bet it will make a difference.
You can check the carbon buildup a couple of ways. Best way is to pull the exhaust manifold and you can see the entire top of the piston. Only other way is to use a scope and look through the spark plug holes. The piston tops will usually be mostly black with some bare metal showing here and there. Usually if you have a carbon problem, you will see it on your plugs too. With carbon buildup you can also see stuck rings, detonation, backfiring, and other things. If you pull the manifold you can check the rings at the same time. It is hard to see them move but there are tricks you can use. I run the piston up until the rings are exposed, then dip my finger in some two cycle oil and smear it heavily on the exposed rings. You can then rock the crank slightly up and down and see the oil squeezing in and out of the top and bottom of the ring as it moves in the groove. Unless you have been using some bad fuel or the wrong type of oil, you probably don't have a carbon problem.
Keep trying, you'll find the answer. Remember Aachem's razor; The simplest solution is usually the correct one.
Bill.
