Yup. It's the gears. Took out the shaft with the chuck and the big gear on it. Closed up the case, so the motor was in the case by itself. Plugged it in and tested it: the motor ran smoothly. Reinstalled the chuck shaft and put lots of grease on the gears. It ran a bit better, but still not well.
You can buy replacement brushes and bearings, but not gears. I'd rather repair than replace, but if parts aren't available there's not much you can do.
