One other thing to check would be where the drive gear connects to the shaft, it might be shifted so it is not consistently square on the shaft. The shaft or inside of the gear may be worn.
I think I would pull the drive motor so it is free of the rack and the drive gear is clearly visible. Then run the motor at a very slow feed rate so you can view the gear and see if you notice any variance in the operation. If you see that there is a problem then you could pull the drive gear and repeat the slow feed rate test to see if it is indeed a bent shaft.
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