Lownslo - That's what we thought originally. At first, I noticed the plane came down and the muffler 'exhaust pipe' was pointing a different direction than when it went up. Assuming it was loose, tried to turn it back and found it tight as could be. A club member suggested as you did, I added some washers. Still did it. He added some washers, still did it. Last time out, the engine was running on the ground for a while, we actually saw the whole back end spinning. The back end spins as does the long screw itself.
Did notice when I took it apart last time that there is a very thin rubber washer/gasket?? (looks like a rubber band) between the two muffler parts. Could that have been larger originally and wore down to where it's not doing its job? Someone suggested sealant from an auto parts store.