Computer randomly restarts?

[Questionable Mark]

Okay, so I have this spiffy computer, but ever since I got it built, it has intermittently crashed on me, whether it be the dreaded bsod, random restart, or such.

These are my specs:

Intel Core 2 Duo e6600 @ 3.0ghz (overclocked)
A-DATA DDR2 800 RAM 4gb 4-4-4-12 (overclocked)
300gb Western Digital 7200rpm HDD Sata 3.0gb/s
evga GeForce 8800GTS (superclocked edition; also overclocked)
Asus P5N-E SLI mobo
550w PSU (Antec)
Antec 900 case w/ all fans on high, and an additional 120mm fan on side panel
Windows XP Professional SP2
I always have the latest drivers.

My random restart and bsod problem has occured even before I overclocked my system. When running stress test for each individual component overnight, I get good results (nothing is unstable).

However, when I try to run a game, sometimes my computer just seems to freeze (the sound stutters) and my computer restarts. That, or I get a BSOD.

I also use bittorrent and crashes seem more frequent if I have used it.

Please help!
Please check back often in case I add new or additional info that may help resolve this.

If you can help, I would also appreciate email correspondence through yahoo answer's private message function.

Thank you.