[ubuntu] How do I find what's causing a system crash?

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http://ubuntuforums.org – Hello, I have upgraded to Ubuntu 12.04 and although I like it a lot ever since the upgrade my system has been sporadically (but consistently) crashing. And the way it crashes is by completely and without any prior warning switching off as if I suddenly unplugged from the wall.. I read somewhere this might be caused by faulty hardware (memory, motherboard) so I ran MemTest86+ for several hours (~7) but it didn't report any errors. Any ideas as to how I could investigate what is causing this issue which has made my system very unstable and unpredictable? Thanks, Savvas (Hardware)