The flags you recommended did not make a difference.

If GPU-Z is running when I start guiminer then there's a lockup. It doesn't matter what comes first, CCC/GPU-Z or poclbm, but they can't seem to coexist.

Also, just like this guy here:

http://forum.bitcoin.org/index.php?topic=22575.0

...the computer also hangs immediately when I start to play a video (say, youtube).

No OC whatsoever. PSU is 1500 watts, and the 6990's 12v inputs are hooked up to separate PSU ports. I can load the CPU with a benchmark while mining and there are no issues, so it's most likely not a wattage problem.