I've toyed with the script and found that in order to disable the the z-jump effect you need to manualy set or tick the 'polygon' box in the models property dialogue.
Why the scripted action doe not work is beyond me at present.

What was happening was that the way it was structured actually made an unlimted jump hieght rather than the default stepping hieght because of the bounding boxes being too small 'within' the models poly bounds.
A conflict resulted which seemed to allow poly collision outside of the hull which produced the z-jump effect.

This also produced the 'sticking' effect you reported.

So, ensure the properties poly box is ticked is all I could come up with.