Mission_control_is_constantly_changing_the_positions_of_windows

I'll bet you have "group windows by application" turned on in Mission Control preferences. If so, try turning that off.

I think the reason why this happens when that option is turned on is because when Mission Control scales the windows down and tiles them, it tries to keep windows from the same application in roughly the same arrangement. So depending on where exactly those windows are placed, it can change how Mission Control tiles them. If you open, say, multiple Finder windows and then move them around to different areas on the screen and invoke Mission Control, you'll see what I mean.