Tanooki pointed something out, I figured you would have known whether or not it was the server since you seem familiar with computers. Do my lan test first, it's in this post... play with bots and see if it happens 
It would be nice to know the amps on that powersupply, although probably not your problem. My suggestion is to reset the bios to default (and switch anything you need on, for example I have to set my ram timings manually).
Start by reinstalling drivers (nvidia, directx, sound card, etc...) and verify the integrity of the game cache from the steam game list, right click on it. Then test the game, if it doesn't work then we begin to rule out hardware.
Here's what you should do depending on what piece of hardware is at fault
CPU = If it's your CPU try underclocking it (Change the FSB and voltages)
GPU = If both of them are broken which would be unlikely, RMA them
MEM = Use only one stick and then use only one stick of the other to see if one of them will work properly, if not RMA them.
HDD = If the HDD cannot be fixed with chkdsk or a format, RMA it.
PSU = If your computer shuts off it would be a sign that your PSU isn't enough, but right now I have no reason to believe it's your PSU, but if it is buy a better one.
SND = Try updating drivers.. if it fails RMA
MOBO = Flash the bios, if it still is broken and you've ruled out everything else try a new MOBO.
Use one video card to rule out sli, like you are doing now, have nvidia configured to use one video card, then verify the game cache file integrity using steam. (Right click it on steam and go to properties), if this issue still happens, one of your parts may be faulty.
It could be anything from a
bad driver in windows, to a bad part, if it was a part from your descriptions my best bet would be your
network card,
GPU, or your
CPU, try hosting a lan server
(Also known as a listen server) and adding bots using the command "bot" and see if it still happens, if it does you can rule out the
network card.
Try swapping your
GPU with the one you took out if you're still finding the problem. If it's possibly your
CPU, run a stress test on it, OC'ing may have damaged it. If it comes back fine, run memtest on your RAM, if that comes back fine, and if it still doesn't work, try the flashing the motherboard with the latest drivers (look up the manufacturer on google), and if that doesn't work the next thing to do is extremely unlikely but..
Time to rule out the
HDD
Go to start -> run -> type "cmd"
Then type "chkdsk" if it comes back with an error type "chkdsk /f" to fix it, this may or may not be related even if you get an error, I get them all the time.
At this point, if none of this has fixed it, you should try another motherboard, get yourself a cheap one at newegg. I have more suggestions, but they only get more extreme and unlikely, come back if you have tried all my suggestions and are stuck.
Edit: Made it much easier to follow, and added a bunch more stuff
Phew, that's probably a lot to read.
PM me on this forum if you need me to explain anything in detail, or post in this thread, I will more likely see your PM first. You can also add me to friends with steam, my steamid is nossr50, but I only run that when I'm playing it. For most of today I'll be gone, my friends about to show up.
Edit 2: I added even more stuff... shame on me
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