FPS Dropping While Playing

[Darkoness21]

I recently build this computer and netplay was running fine at 100% FPS. Now the FPS drops every 30 secs from 100 to about 64% even when I play single-player. Is there a way to fix these FPS issues?
(Asked the same question in the guide thread but decided to make a new one in order get some help)

Here are my specs:
Processor: AMD Athlon (tm) X4 760K Quad Core 3.80 GHz
RAM: 8 GB
Graphics Card: NVIDIA GeForce GT 640
Windows 64