Window screen will not stay up

On our first trip with less than 40° F temperatures we found that the passenger side screen (bottom mounted) does not stay up when you want it open an inch or two for ventilation. It has worked fine on other trips this spring and summer where lows were above 50°. It does seem to be temperature related because after we got home it worked fine during the day at 75° to 80°, but would not quite stay up later in the evening at about 50°. As best I can tell there is a gray U-shaped piece of plastic or rubber on the ends of the sliding aluminum frame that uses friction to hold the screen in the desired position. I'm guessing that colder temps cause contraction and loosening of this piece. I thought I saw a post months ago talking about this problem, but I have not been able to find it. The front window screen behaves the same way, but the driver side (which we generally use less than the others) stays put wherever you want it. Any fixes for this? (other than resorting to using a small spring clip.)