I know at least one other person has had this problem, and since I found a solution, figured I would post it.
When you roll the windows up and down, the crank moves a long arm with a pin on the end that goes through a groove in the bottom of the window frame and is held there with a clip...it isnt exactly like any other clip I know of and I havent been able to find one anywhere. For some reason the one in my passenger door is weak and pops off occasionally, letting the window drop to the bottom of its travel very quickly without warning...not good!!
Well I took the arm off Saturday and took it to NAPA and went through their C Clips until I found a good replacement. This clip fits TIGHT in the groove...hard to impossible to do with your fingers...luckily, if the window is about half up/half down, I was able to get a stubby screwdriver in the door and put it in the groove that the arm slides in as the window goes up and down. By pushing sideways, I seated the clip pretty easily....does take some force and certainly is not coming out without effort!!
The part number is 1009 at NAPA and cost a whopping $0.49!!
Hope this helps someone else.
brute4c