Random Shutdown while Transcoding

AMD Phenom II 3.4GHz Quad-Core Processor
16 GB RAM
1.5 TB Weste Digital SATA Hard Drive
ASUS GeForce GT440 1GB Video Card
Hauppauge HVR-1600 Tuner Card
Creative Sound Blaster X-Fi Card

I am going through and manually transcoding all my saved TV episodes to permanently remove the commercials as per this site: https://www.mythtv.org/wiki/Removing...g_mythfrontend

Sometimes it works fine, sometimes halfway through it completely shuts down my computer without waing.

My first thought was obviously overheating. I've blown it out with compressed air already. Looking at psensor, though, the highest temperature on any processor is ~65 degC. When it transcodes without shutting down, it can handle this temperature just fine, then cools down again once transcoding is complete. Further, when it does shutdown, the temperature is usually not above ~55 degC.

Once it shuts down, if I try to restart right away, it won't make it through the startup routine without randomly shutting down again - again making me think overheating, but psensor just doesn't support that.

Thus usually happens after the computer's been idle for a while. Top and iotop don't show anything out of the ordinary, though. Top will show >100% cpu usage during transcoding, but this is not determinate for shutdown - sometimes it handles it just fine, sometimes it shutsdown.

HELP!