Peter,

The _part_ that is flexing is the entire frame peice that makes up the upper part of the door. If I roll the window up with the door OPEN, I can watch the top part of the door frame twitch/flex. I opened another door and rolled the window up to see if this behavior was repeatable/normal. Didn't happen on the other door.

So, I've just concluded that this behavior is because the window it trying to go up further than it's suppose to.