PC shutting down under heavy load. Is it my PSU?

Hey guys. I'd like some other opinions on this before I run off to Newegg with debit card in hand.

I've recently upgraded my PC so I can play some newer titles, such as Battlefield 3. I purchased a new quad-core CPU, an upgrade from my dual-core, and GPU (see specs below). I was having some trouble getting the performance I expected out of the cpu, but I saw that my BIOS was not fully updated and I was able to unlock all four cores after an update.

See: http://www.tomshardware.com/forum/120432-13-trouble

Everything seemed to be running smoothly after I upgraded the BIOS, but now my PC will just shutdown in game. The screen will go black, and I can hear all of my fans shut off. This only appears to happen when the system is under a heavy load while gaming, and not while I'm just surfing the web or watching movies. I've been checking my temps and everything appears to be within a reasonable range. All of my drivers are up to date now, and I don't believe something like a faulty GPU driver would necessarily lead to an instantaneous shut down of my entire system.

After ruling those couple of things out, I'm thinking that it might be the PSU. It's a 750W PSU that I bought a couple of years ago, at the time it seemed almost excessive, but perhaps I need an upgrade for my system to work properly with the new components. My PSU is also powering some external components such as an external hard drive, USB mouse, keyboard and headset.

What do you guys think? Thanks in advance for the input  

My specs:

500 GB HDD
Motherboard: Asus M4A77TD PRO AM3 DDR3 AMD 770 Chipset
PSU: CORSAIR CMPSU-750TX 750W
CPU: AMD Phenom II X4 975 Black Edition Deneb 3.6GHz
GPU: GeForce GTX 560 (Fermi) 1GB 256-bit GDDR5
Memory: 4GB (2GBx2) PC1333 DDR3 PC3 10666 Dual Channel Memory

My PSU: http://www.newegg.com/Product/Product.aspx?Item=N82E168...