Are you saying that you guys have been running the same engines for three years? If so, you're doing pretty good. I could tear up an engine in two months, or less.

If the engines are that old, I'd pull the carb and make sure that the o-rings are sealing. For some reason, more air than is supposed to is making its way into the engine.

Have you pulled the head to make sure that the gaskets are sealing? When you say that there is good compression, did you try turning the motor over with the plug removed? Are you sure it's compression that you're getting and not pinch?

I don't know, I'm just throwing things out there. Trying to diagnose an overheating problem without seeing the engine is like calling a mechanic on the phone and trying to describe a noise to him.