System Randomly Shutting Down

Just registered here, so hopefully this is the right place to ask this. I built a new rig about 2 months ago, and everything has worked perfectly up until yesterday. Since then, my computer has randomly been shutting off/restarting itself. Sometimes it will shut off and then restart back up on its own without anything else happening, and it worked fine. A few times it has shut off and tried to restart itself, but the CPU LED on the motherboard stayed red and my monitor never picks up a signal, causing me to have to restart it again manually.

I haven't installed any new hardware since building it, nor has there been any common trend or thing I was doing when the shutdowns happened. Using SpeedFan, my CPU temperatures have never been above 20-25C when the crash happened, so it doesn't appear to be overheating. I've run virus/malware scans and nothing was picked up. There have been about 5 or 6 shutdowns since yesterday, some being like an hour apart and others like 10 hours apart. Any idea what could be causing these shutdowns?

Specs:
My build is pretty much exactly the 2nd "Gamer" build listed here:
http://www.tomshardware.com/forum/ [...] e#t1968923 (the one with the i5 and GTX460)

Any help would be appreciated a ton

1. Remove any overclocks.
2. Make sure RAM is manually set to correct timings and v per manf specs in Bios.
3. Run memtest for at least 8 passes.

Most common causes for this is bad RAM.