I got a computer only 2.5 years old which has: 2.4ghz intel CPU, 512mb
pc2700 ddr ram, 9800 pro radeon video, 40gb hd, cdrw and dvdrw. Recently
its been getting this odd behavior where in 3d games it runs for 3-4
minutes fine, then it stutters/freezes for 2-3 seconds and then runs
fine again. This loop continues on for the same time intervals almost.
This happens in every 3d game. I dont notice it in windows while
watching videos or 2d games.

What i've done without any success is:
-Exchanged video card with an older one
-Defragged countless times with third party programs even
-Cleaned registry and used diskcleaner software
-Ran all type of disk scand and surface scans
-Ran memory tests like memtest86
-Ran MS antispyware, Adware, Spybot
-Put in a new more powerful 420w power supply
-Updated all device drivers, windows
-Unplugged all usb devices
-Disabled or closed all programs and processes i can when playing a game
-Check temperatures look fine

I don't know what could be causing this or what else can I do? Reinstall
windows would be my last step?