Jack,
I think the problem lies in the collimator body being loose in its support frame,having a bit of play between the threads the body is being pulled toward the motor side of the instrument and in doing so the line is being pushed towards the edge of the achromatic lens. Hopefully adjustment of the collimator screws will be enough to alleviate the tilt with just enough tension to allow rotation but at the same time keeping the collimator body square to the grating/camera sensor.
Regards
Alun