[...]Is fglrx the proprietary ATI driver?
Yes it is.
If you have some fancy (opengl) screensaver, try disabling it. I've seen quite a few posts on various maillist about system freezes and screensaver/gfx-cards.
No, no fancy screensaver installed, just the default one which turns off the screen after some time (works fine). Also I was working while the freezes happened so this shouldn't be the cause.