Binary package hint: compiz
I have an ATI Radeon Mobility X300 graphics card. This card is known not to work with compiz at this time, and is listed in the !cfblacklist ubotu factoid. However, after a completely clean vanilla Ubuntu (Gutsy) installation, desktop effects are enabled. This leads to hard lockups of the entire system randomly, after about 10 or 15 minutes of use I'd say, recoverable only through the kernel magic SysRq keys or the power button.
There does not seem to be a particular pattern to when the lockups happen, and they will occur even if a fairly minimal set of compiz plugins are enabled, and nothing special is being done at the time (just reading pages in Firefox, for instance). Disabling desktop effects fixes the problem. This seems to happen with both the open-source and proprietary driver.
I know that a blacklist of cards was added for compiz, so either it is not obeying that list or this card was mistakenly left off of the list. The X300 simply does not work, and should not allow desktop effects to be enabled in the installer. It is debatable whether users should even be able to manually enable it afterwards - much less have it be default for a broken card.
Note: I use the alternate install CD.
Can you attach the output of running "compiz --replace"? Your card should be blacklisted - it's possible that it's got a different PCI ID or somesuch.