Troubleshooting Spontaneous System Shutdown

Very likely having a hardware failure which is causing infrequent, random system shut down.

OS: Linux 3.11.10-21-desktop x86_64
System: openSUSE 13.1 (x86_64)
KDE: 4.11.5

AMD Athlon 64 X2 Dual Core Processor 4800+ / 2Gb RAM cause this aging ASUS A8N32-SLI Deluxe is a royal pain in the butt about running 4Gb RAM.
Powered by an Enermax EG851AX-VH EPS 12V 660Watt Power Supply which has got to be a good 5yrs old at least, and probably older. I tend to extract every ounce of useful life I can from things...

Question 1 is... what log files might I look at that would reveal a sudden and spontaneous shut down event?
And might they reveal my culprit or ...sigh... is this just going to be an easter egg hunt?

Question 2 is... what s/w tools do we have to monitor processor temps? 'twould be nice to have something that would print a history of the processor temp over time. Maybe correlate a spike in temperature with a sudden shutdown event... I can dump the system at any point and run the BIOS to see what the temp says but that's kind of pointless I think.

Thanks for your consideration!