My driver's side door window fell down in the door. Removing the panel I saw that it was just the little metal tang popped out of the round plastic piece which rolls in a guide at the bottom of the window. Popped it back in. Fine. Until the next day it popped out again. Like an idiot I did this two more times.

Finally last night I decided to figure out what's wrong. I popped it back in but left the door panel off so I could watch what was causing it to get stuck and pop off. I opened and closed the door, slammed it, revved the motor (to get vibrations), pounded on the door. Rolled the window up, down, fast, slow, part way, all the way, all combinations. I must have spent an hour and operated the window probably 100 times (probably about a month's worth of window action on the road). It worked perfectly. So I buttoned it up.

This morning I went for a drive and when I tried to roll up the window it popped out again.
(IMG:style_emoticons/default/headbang.gif)

What is going on and how do I fix this?

Thanks,
--Dave