Massive FPS drops on a decent computer?

OS: Windows 8.1 Professional (x64)
CPU: Intel Core2 Quad CPU Q6600 @ 2.40GHz
Graphics Card: GeForce GTX460 (768mb)
Motherboard: P5Q-E - ASUS
RAM: 6 GB
PSU: CoreXStream 500W

For the past few months I’ve been getting massive frame drops in GMod ( and Source games in general. ), I can play GMod with everything maxed out at a stable FPS of 120, but I often get huge frame drops for no reason ( Same happens when looking at an are with alot of props, a large map etc. but thats understandable. )

My brother who has a pretty crappy laptop from 2005 ( I’ll post the specs whenever I get home. ) can play GMod no problem, even when he has a billion props, or looking at a large map etc. same goes for TF2.

I’ve tried reinstalling GMod, Steam, Windows, defragging, various fps configs and many other things.

If anyone knows anything I could to, thanks in advance :slight_smile: