Spin the drum by hand, unmounted to check the rollers, and particularly to see that the holes the nylon rollers ride in are still round, and that the roller bases are fully seated. Also check the magnet ring on the bottom of the tank to see that it is fully seated and flat all the way around. Once those are out of they way, then try shimming the coupler you have to see if you can keep it from moving when tightened. You've checked the coupler for cracks and wear?