Kind of went through the same procedure myself a couple of years back. When ours died I put in a new gearbox, then had to turn around and replace the motor. Reality is it would have been not much more expensive to have replaced the entire thing, and gone up a size to boot. Such it is.
I wonder though why it is jamming ? Have you had a good look at the shaft ? Plenty of lubrication ? I did find when I pulled ours apart back then that the grease nipple was in poor shape and may not have been doing its job. In our case that meant water getting in which was what ultimately ruined the gearbox but maybe just maybe you are not getting enough grease onto the shaft ?
Sorry, not much help I know. This was more to provide some moral support.