I would post your XML and someone can look to see if there are problems there. The problem sounds vaguely familar.

(;-) TP

Are you sure your power supply is adequate ?

Russ

Are you 100% sure BOTH motors are actually turning under power?

I had a similar issue when building and the gantry was being pulled along by one side which would then cause the other side to spring forwards every few inches, sort of like how you explain it.

I would pull both motors and set them side by side on top of the table, fit a marker - tape or pointer etc to each one and run some sync tests.

found the problem was the shafts in the reduction pulley would turn almost a full turn then catch and then release. Know the opposite side is doing it. I dislike to have to use set screws to secure to a shaft as this would happen. And yes I did flatten were the set screw hit the shaft. First time setting pulse for movement right on the nose.

It sounds like mechanical oscillation