For those interested - while I was having this issue on 10.04 (before backing down to 9.1) I did disable acpi by booting with acpi=off. That did nothing and the xorg process still took over with 100% after varying amounts of time.
I had also disabled any compiz effects (which I have to do anyway because you can't vnc into the system *witth* compiz) and the screensaver.
For those interested - while I was having this issue on 10.04 (before backing down to 9.1) I did disable acpi by booting with acpi=off. That did nothing and the xorg process still took over with 100% after varying amounts of time.
I had also disabled any compiz effects (which I have to do anyway because you can't vnc into the system *witth* compiz) and the screensaver.