Originally Posted by EvenStranger

Krax - sounds as though your video processor might be overheating and causing the system to crash. If the computer is under warranty, I'd take it to an Apple store or repair center and have them evaluate it. It sounds as though the problem is easily replicated by playing a video.

Petrushakaml, your issue appears to be different. Yours looks as though it's an issue with your Core Software. For you, I'd download the 10.6.7 Combo updater and run it to see if the issue smooths out with updated software. Ultimately, it's possible yours could require a full reinstall of the OS.