Mike,
The pitch almost never changes on its own. I've never seen that happen. You have checked everything that would normally cause this problem. Rads are of sufficient size and are clean, water pump is OK, compression is acceptable, and piston tops and cylinder walls look OK. I would go ahead and inspect the rings through the exhaust, but based on your compression check and scope inspection findings, I suspect you are unlikely to find a problem other than normal wear.
I'm gonna clutch at straws again here. Are the EGTs good at WOT. Sometimes a weak fuel pump won't supply enough fuel get the engine to its top RPM. This usually results in a lean mix, reduced RPMs, and high EGTs at WOT. Are the EGTs normal at WOT? They should be somewhat higher but not abnormally higher. Is the fuel filter clean? You can get the same problem there. If you have a strainer in the fuel tank, it could be partially clogged. Has fuel consumption gone up?
That engine gets its best fuel economy between 5000 and 6000 RPM. Have you notice any drop in RPMs necessary to maintain given cruise speed?
I think you are right in thinking that it is not a prop problem, but if all else check out OK, I would give a pitch change a try. Too much pitch can cause lower climb rates, cruise RPMs that are too low, and overheating. Too much pitch is kind of like trying to run a car at low speeds in high gear. It overworks and overheats the engine.
Hell, at this point I'm willing to try waxing the the windshield. I'm sure you are too.
Bill.
