So here's what I found. I took the regulator/motor out, and noticed that when I did my zip-tie repair a couple of years ago, I seem to have left the heads of the zip ties in a position where there's sticking out far enough to bind up the regulator mechanism. Thinking I had it solved, I removed the zip ties, and installed new ones, taking care that they ended up in positions where they wouldn't hit anything as the regulator moved.

So I put the regulator back in, and ran it up and down before I attached the glass. No problem - very smooth, fast operation in both directions. So this definitely ruled out the motor as the culprit. But when I tried re-attaching the glass, I ran into a major problem - after struggling for nearly an hour, there was no way I could get both the front and back clips (the ones that hold the glass to the regulator) to simultaneously align with the positions in the piece of the regulator to which they attach. So, my theory now is that somehow, the regulator has become "non-symmetrical" - maybe something internally stretched, wore, or moved in some fashion on one side of the regulator, creating a situation where the front and back parts of the regulator are out of alignment relative to each other. I can't visually see anything wrong with it, but I'm pretty sure the "non-symmerty" is what was causing it to bind as it was trying to raise the window. So, unless someone has any alternate theories, I'm going to go ahead and order a new regulator.

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2003 X5 4.4i
1992 Porsche 968
2007 328i
2014 Subaru Forester XT