Shutting Down Under Load

Recently I did a bit of an overhaul on my old gaming computer, upgrading from an Nvidia 9600 GSO to a GTS 250, and overclocking the processor speed of the old Dual Core E2200 processor from the typical 2.4 to 3.2 GHz. Life was pretty good for a couple days, but then it started shutting down under heavy load. I couldn't have that, so I dropped the clock back down to the default 2.4. Everything was fine.

Eventually though, the motherboard and PSU failed, and I took the opportunity to upgrade with the rebuild. So now I've got a Phenom II x2 555BE on what seems to be a solid, if no-name board. The only parts that haven't been changed out are the graphics card (my beloved GTS 250), and the RAM sticks (corsair 5-5-5-12 sticks at 1.8 volts). The temps have been good, the computer is blazing fast compared to what it used to be -- and it shuts off under load again! So what do you think, Tom's? Time to test the old video card out and run some tests on the RAM? Maybe increase voltages a little? I'm writing this as I go to bed, but I'll be sure to respond in the morning when I've had some time to sleep.

Edit: I thought I should mention that I'm still running the same programs I ran with the E2200: TF2, Heroes of Newerth, a couple other simple games, and then the big boys, STALKER: Call of Pripyat (complete mod) and the Crysis 2 multiplayer demo. Both of the latter two crash within 20 minutes of play (if that), and are intensive on all parts of the rig. I mostly suspect the GPU because TF2 can be reasonably demanding on the processor, but significantly less so on the GPU, and has never given me a crash like this, whereas Half-life 2 has in SPOILER the infamous G-Man scenes.

Second Edit: System specs
AMD Phenom II X2 555BE processor
2 2 gig sticks of Corsair 5-5-5-12 RAM
Nvidia GTS 250 gpu
Logisys PSU, stable at 450W. Part # PS575XBK
abit N11360 mobo
Western Digital 5000AAKS HD