Z axix peculiar problem

Hi,

I have built my own prusa i3 a few months ago, It was all set up and printing well. I took a few weeks break from printing to built an enclosure for it.
I have just finished building the enclosure, I have the printer inside the enclosure with my arduino/ ramps board and psu all mounted to the back side of the enclosure.
I have plugged everything back in and checked the wiring, it connected to pronterface fine and I started checking my axis, x moves correct y moves correct then i checked z, it appeared to be correct but then it jammed.
I reset the z so both sides were equal distance and tried again, worked fine then jammed. On closer inspection I realized that my left z axis stepper moved correctly but my right axis stepper motor would spin in 1 direction and then change and spin the other way.

Has anyone encountered this? How can I fix this?

Thanks.

Do you have the motors wired in parallel? Or are they wired in series? If they are wired in parallel, have you checked that everything is wired the same? If they are wired in series, have you checked the circuit to make sure it is correct?

Have you checked the drive current settings? (Is there enough power going to the motors, because if not, you maybe missing steps and that can cause it to reverse direction for a short while.) Do both Z-Shafts turn easily? Or is one of them harder to turn?