I wonder if my XTR M952 shifter has this gumming up problem. Between 3rd and 4th gears, it doesn't seat all the way to 3 when shifting down. So that eventually it snaps into place. This used to be a bigger problem but I opened up the shifter and greased it up. Since then, it never snaps into place, but rather just doesn't fully seat correctly. So it just rubs on the cassette a bit, which I hear while pedaling, so I just push a bit more on the shift never which seats it into correct position. I will take it apart again, looking for this caked up old grease. I just assumed that the gear mechanism inside was just a bit too worn due to lack of grease.