Reply by ChrisFranklin

Bit of an old thread, but in hopes of helping somebody down the line: these sanders have 3 ball bearings. Two are for the two ends of the motor shaft like any other motor. The third is between the flywheel/fan and the sanding pad, and is slightly off-center, hence the random orbit. If this bearing seizes the RO sander becomes a high-speed direct-drive grinder. Sounds like this might be the problem in the OP. It’s part #23 in the drawing linked above, 4-5 bucks to replace.

-- Mud thrown is ground lost.