It is hard to tell from here.
Are you saying that you turn it on, and it works fine for a while, and then (with the power on continuously) that it shifts about a 1/4" and stays shifted after that? Maybe there's a mechanical issue and it is sticking on something and skipping steps.
I think Paul is saying that powering off or dis-ENABLEing the stepper driver and re-enabling it could cause a shift to a different detent position. Is power on continuously between startup and the when the problem is observed? Or does the shift happen with some power interruption?