IMO... something indeed is "hosed". But before you put a new gearbox/motor in you need to check a few things.
I hear the motor spinning really fast and the gear box jerking along. That's a combination that does not happen.
If the gear box is good and you stop the motion, the gears will break under all circumstances. It's direct drive, there's nowhere for the energy to go.
If the gear box is broken, why does it keep moving?
Check the path of the MPF completely on all four sides. Make sure it's 100% free of any binding/rubbing.
Use my white paper to tear down the whole thing and build it back again. Just the physical parts (leave the top of the MPF alone).
When the brackets are out, you need to run them with the motor alone up/down to make sure the action is smooth. If they got hung up, they warp/twist. You may need to reshape slightly.
I'm wondering if the set screw on the cam is loose or slipping. This would be a simple fix if that's the case.
Another possibility is one of the gears inside the gearbox has sheered off it's mount. There's some delicate repairs to repair the gear collet but unless it's done correctly it could turn into a lot of wasted time/frustration. But if you love to try to fix things, this would be a good one
Let me know. Glad to call you too.