See these FAQs as well as the general instability FAQ which xbprm and michael mentioned:
[ul][li]Crash, exit or high CPU when viewing a certain directory
[/li]
[li]Crash, exit or high CPU usage when right-clicking certain files[/li][/ul]
(I've just added links to those two FAQs to the general one as they're all related.)
My bet is that you've got video codecs which are falling over when you visit folders with certain video files. It could be a lot of things and I might be wrong, but that usually seems to be the cause of apparently random instability like you are seeing. The "when viewing a certain directory" FAQ will help find the problem if I'm right.