Out of the blue this happened: The printer start is perfectly normal and at the time the actual print of the part starts the feed roller snaps backwards in between forward movement.
Do you have any more information you can share like exact model. Also, it’s hard to interpret what you are saying. Maybe you can rephrase it or have a video showing the issue?
Here is a link to the U-Tube video I uploaded.
It’s slipping from possibly a stripped screw or thread. Did you try tightening?
Or the stepper motor cable might have developed a fault, I would source another cable after checking those screws on the pinion.