K,.. here are DH's suggestions:
1. We discussed the motors being swapped, so this might not be relevant: First place heat will start to be generated is in the bearing on top of the motor, is it lubed? - of course if you changed the gear and motor together, this is irrelevant, and I believe you did. It does seem like a bad / hot bearing type of thing though...
2. Definitely take a peek behind the fiber gear. Because of the spring action, maybe it's malfunctioning somewhere.
3. See if you can prove the shaft is bent, with a pencil and a piece of tape or something. See if the mark it leaves is "straight" or if it wobbles.
I think that was all of them.
Get some rest. We don't troubleshoot well when we're tired or sore. Sewing machines are like computers, they don't care if they're not 100%, we do.