Ubuntu freezes randomly, only mouse works-8.10

I have been having this problem for some time. Ubuntu freezes suddenly, I can only move the mouse and the only way out is a push on the power button, when everything comes back to life. Please help !!

thanks

Ubuntu 8.10 i386 Desktop freezes. Mouse moves but cursor is stuck on the hand icon instead of the pointer icon. Clock continues displaying the correct time. Just changed the operating system to ubuntu 8.10 from CentOS 5 which ran very well for months... Mentioned CentOS simply to rule out PC hardware failure.

EDIT: Ctrl-Alt-Backspace brought up the login screen. After being frozen for roughly twenty five minutes or so, it is possible to recover without hitting the power switch. However, all jobs running when ubuntu froze were of course not recovered.

NOTE: Ctrl-Alt-Backspace is not Ctrl-Alt-Left Arrow Key :-) Oops!

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