The tumbler in question is the Harbor Freight 5 lb. model.
The motor lubricant had separated, I guess, so I took it apart and oiled the bushings (top and bottom) with machine oil. This freed up the motor, and it spins freely now, but the motor cuts out after about 45 minutes, and it's very hot (perhaps there's a thermal cutoff?). Had to let it cool down for an hour, then it started up again, but after 30 minutes, I can feel the heat building again.
I have no confidence that this tumbler will work for any duration, let alone not cause a fire.
Thanks for attempting to troubleshoot, Vid!