If the window flaps in the breeze in the sense that it no longer resists turning, it's probably toast. The correct stiffness in the window is produced by a spring in the bottom pivot. When the mechanism rusts, the pivot resists rotation too much and either breaks or comes loose where it is staked into the bottom frame of the glass. Look at yours, and I'll bet you will see that the bottom pivot post is standing still and the frame is turning on it, where it should be turning the post. You might be able to open the whole thing up and loosen the pivot spring, etc. but your problem would be reattaching the pivot to the frame without damaging the glass. I don't think you can weld or braze it, and I doubt any kind of adhesive would work, and you can't safely remove the glass from its frame - it's very tight. I have puzzled over this in the past, and ended up replacing the window.
Klonestar, if your window is becoming very stiff, I suggest you promptly get in there, remove the window and service the bottom pivot before it breaks. If you take out the window you'll see at a glance how it's put together. The bottom post goes down through the frame, and has a nut at the end, which holds a rather stiff cooil spring for tension. You can take that all apart and lube it and reset the tension to taste. If you wait too long and the pivot becomes unstaked from the glass frame you're out of luck.
