Comment 8 for bug 477972

Pierre, on some other ubuntu freeze bugs on 855GM it is reported that booting with the kernel parameter acpi=off works around the problem (Esc at boot to show the Grub menu, 'e' to edit the default entry, add acpi=off after quiet splash, Ctrl+C to boot IIRC). Could you check if this is the case for you also?

Another tests I would like you to do:
- Check whether it occurs without KMS, that is, instead of acpi=off, use the option nomodeset

There are a couple of more options that you can try in xorg.conf at https://wiki.ubuntu.com/X/Troubleshooting/Freeze#Narrow%20Subsystem%20it%20Occurs%20in . If any of them stop the freezes or reduce the frequency of them that is also a useful data point, since it may tell us which part of the driver is responsible.

Finally, there's a chance that a newer kernel might have some fixes that resolves this. You may get newer mainline builds at http://kernel.ubuntu.com/~kernel-ppa/mainline/ and there is some more information about those builds at https://wiki.ubuntu.com/KernelTeam/MainlineBuilds .

Even if none of the mentioned tests changes anything, that is a very useful thing to know.