System Freezes

[Daytraders]

My computer started freezing occasionally when playing World of Warcraft. It happens about once or twice over a 4-5 hour period but is about to get me kicked from my top 200 guild.
Since it started happening, I've replaced the following parts:
-Case Fans
-CPU
-Motherboard
-Hard Drive
-RAM

The only parts that are still the same are:
-Video Card
-Power Supply
-Case
-Disc Drive

Despite the changes, my computer will still freeze. Based on what I've replaced I believe the issue is one of these:
-Video Card Overheating
-Not enough wattage from PSU when Video Card is under stress
-CPU or MOBO Overheating

These are my system specs:
CPU: Intel dual core e6700 @ 3.2 ghz (not overclocked)
MOBO: Gigabyte G31M-ES22L
Video Card: EVGA GeForce GTS 250 1 GB
PSU: Ultra 400 watts
RAM: 2x Ultra DDR2 800 mhz, 1 GB
Hard Drive: Western Digital Caviar Blue 160 GB
Operating System: Windows XP

Forgot to mention, when I check the event log after my system freezes there are no errors.

 

[rolli59]

I would start by replacing the PSU its size is just enough for your setup. It is recommended to have 450watts with 24amps on the +12volts for the GTS250.