I would suggest to dissconnect the motors from the drive line, the easiest way to do this is to pull the pinions off, and make sure the drive line is very free. Try spinning the props with your fingers and make sure there is no resistance. Then pull the shafts out and make sure they are not bent. If all this is ok, then I think you have a binding out drive where the drive knuckles are. Replace them with the larger ones, I think graupner makes them.