[all variants] Randomly freezes completely

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http://ubuntuforums.org – Hi all, I built a computer a few months ago. Nothing high powered, but a basic desktop. Randomly, infrequently (usually maybe like twice a month), it'll completely freeze such that it won't even allow a soft boot (ctrl alt prt scr does nothing) and requires a hard boot. I first thought RAM. I did some memtesting and found nothing. I've alternately removed each stick of ram and it happened again. What could this be? Is there a diagnostic I should run? I'm thinking maybe it's the mobo. Specs: kubuntu 9.04, up to date AMD Phenom X3 Gigabyte GA-MA790X-UD4P OCZ Reaper 2x2gb EVGA GeForce 9800GT 512mb (Hardware)