Lew, a couple of things come to mind. It sounds like the engine is going lean. You have probably tried the obvious which is opening the idle mixture screw an eighth at a time to see if there is an improvement.

The other possibility is that the air flow across the carb is causing the atmospheric pressure sensor diaphram to make the carb lean out. If you want to try a fix then remove the cover (4 screws for Walbro carb) from the carb. Solder a small brass tube into one of the holes on the carb. If it has another hole, seal it. Reinstall cover. Run a piece of fuel tubing from the brass tube through the firewall to an area of dead airspace inside the fuse. This should stabilize the pressure changes to the carb.

Ken