I have had a similar problem in the past, and this weekend it happened again. I found that the gears on the two parts of the regulator that control each side of the window skipped several teeth. This is because the stamped metal that the regulator is made from is relatively thin and can flex, which allows the teeth to momentarily become disengaged. There is a little metal tab on the backside of the gears that is supposed to hold them in plane with each other. After wrestling with mine for a while, I was able to get it straightened and the proper teeth engaged. I feel like this is a poor design. To help reinforce it, I drilled a 1/4 inch hole through the tab, and threaded a bolt through it with a large washer and a piece of short bar stock on the front side. When tightened down this helps to pull the tab down against the back of the gears to keep them engaged. I have not fully tested it yet, but it should be a relatively permanent fix. I am afraid that a replacement regulator could have the same issue unless changes were made to strengthen this area. When I have time I will test it some more and take some pictures. Right now I am trying to figure out how to reset the limit switch that shuts the motor off when the window is at the top of its travel. Still trying to figure out how this works. If anyone knows, let me know.