Comment 40 for bug 447892

kalessin, at bug 476817 which is another freeze bug on 855GM, it is reported that adding acpi=off on the kernel command line makes the freezes go away. Could you check if this is the case for you as well?

You other guys, we know that you also have freeze problems, but please don't report your own findings here. It is already difficult to filter out what symptoms kalessin has, and it gets more difficult the more people posts here. Please only post here if you have suggestions for what kalessin may try or it is otherwise relevant for his case. If you are totally convinced that you have the exact same bug and want to report your findings, it is actually better if you file your own bug report and mark it as a duplicate of this.

There are several freeze bugs on 855GM and those that have been tagged with the chipset name (855gm) and symptom (freeze) show up in this report:
https://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-intel/+bugs?field.tag=855gm%20freeze&field.tags_combinator=ALL (see https://wiki.ubuntu.com/X/Tagging for details on tags on xorg bugs).