ET screensize problem?

I've got 1280x1024 set on my desktop. If I set a higher one in my config, for example 1920x1080 (irrelevant why I do it), part of the screen in the game comes out. Typically, the upper or left of the screen, then you can not see the top / left side of the menu in the game, console, etc.. Sometimes, however, the picture is stretched and fits the screen, the game elements are visible. Anybody know exactly why this happens? Without changing the settings in the drivers, refresh rate and so on. It sometimes works, sometimes not.

chick :P