Had same problem last year, tried replacing and re-engaging..worked a few times then went down too far again Tried a few things but ended up buying a new regulator. What I didn't renew was the bottom bit that grips the glass but wished I had as it sort of bent and twisted itself, caught on something and my new regulator got mangled too. How I larfed. There should be posts on here about renewal, it's a bit fiddly but not difficult - I got real good at it after taking it all out about 8 times!
Just remembered, if you keep the one you've got and re-engage it, put a bit of wood or similar in the bottom of the door to stop it going too far! Cheap and cheating fix... a temporary/permanent sort of fix!
REME Field repair for the same - 1 x 14" long lump of 3"x2" pine... slip that in the bottom of your door and no more problems!
On a more serious note (although the above does work - well) check the spot welds on the regulator... mine started doing this and three weeks later the quadrant gear fell off completely! Not sure but I think the overrun may have been caused by one or two of the three spots being broken. The regulator is weld repairable in this case...