Massive FPS drops on a decent computer?

[Ryuko]

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