Comment 9 for bug 446421

There have been several reports on the forums (mostly on r600 cards IIRC) about random system hangs with ATI and KMS in lucid. Also I have been helping 10+ people with these problems.

The symptoms: The machine hangs all of a sudden up to multiple times a day. What seems to trigger this is very often playing a flash video or also playing other videos or playing games or sometimes doing nothing at all.
There's nothing whatsoever to be found in syslog or all the other logfiles I've looked at so this is a very hard one to catch.
However there is a fix for this problem: using a -34 series kernel (which, of course, is still in its rc stage right now). With these kernels the machines run without problems. Mine (ATI 2600XT) used to crach twice a day to every second day and it's been running stable for over a month now using the -34 kernel series. This also fixed all other machines.

So my take on this is that there's some sort of problem in lucid's DRM stack and judging from the amount of machines affected I'd say this is definitely a critical bug.