It is almost certainly the grism holder that that has come loose and is rotating relative to the slit. (It seems to be a quite common problem. Not sure why but I suspect Shelyak dont always tighten them up properly when the leave the factory as I have never had one move on its own after tightening. It is easy to fix though. The grim is held in the core module by an allen headed grub screw(s?). You view the sky spectrum for example visually through the core module, and rotate the grism until the spectrum is square and tighten. It is worthwhile getting it as precisely square as possible otherwise the ISIS software automatic wavelength calibration can fail. You can see a picture of the assembly here (page 4)
Take care if you slacken the grub screw off too far ,there is a spring underneath the grism assembly that holds the collimator lens in place which can launch everything if you are not careful!