power sliding door, and pull on '05 caravan

The right-side power sliding door on my '05 caravan stop in the middle of the track whether opening or closing under power. I cannot
see anything impeding the movment, nothing wrong with the plastic 'belt' that seems to be what motivates the door (can't see anything else that would) and I can't feel any 'hitch' or blockage in the movement if I move it manually. If I manually move the door past this point, it can be made to continue under power.
Any ideas what would be causing this or what to do about it?
Also, after having 3 independant shops look at the alignment and 2 dealerships, nobody can tell me why this car develops a continual pull to the left. Any ideas here?
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