Well, maybe Catalyst 14.4 RC fixed the fog effect, if this was really the cause. Either way, I was able to see the fog effect with 14.4 RC without stuttering and freeze. My PC did freeze, though, but I just don't know atm, what is doing it. It's all happening in the "press any key-menu" or shortly after I press any key or click with the mouse.
I also have this problem; Metro: Last Light freezes within the first minute of starting it up. Actually, it outright shuts down my computer without warning.
Somewhat similarly, while exporting video on Windows 7 in Adobe Premiere (at 1080p for YouTube), my computer also shuts down.
Ubuntu 12.04.4, 64-bit
Unity Desktop Environment (vanilla)
Asus M4A78LT-M LE Motherboard
Asus BIOS Version 0803
AMD Athlon II X4 645, 3.10 GHz
AMD Radeon HD 6790
Catalyst Control Center Version 2.20
ATI/AMD Proprietary FGLRX Graphics Driver (**Experimental** Beta)