Very low fps

When I got Gmod, I had about 10-20 fps on a Nvidia 9600, then I got a nvidia 9800Gt and got about 30-40 fps. And as of now I have a Nvidia GTX 260, which cost about $150 more than the last card plus I had to buy a new power supply for it to run.

But the problem is that over the past month or so my fps has dropped from about 30-40 fps to about 9-10 fps. And it refuses to go any higher. I just really dont know whats going on,

Maybe it has to do with my specs. So I hope someone can help.

OS: Windows 7 home premium
Processor: AMD Phenom 9550 Quad-Core Processor (4 CPUs),~2.2GHz
Memory: 5120MB RAM
DirectX Version: DX 11
Video: GeForce GTX 260 (3053 MB)

I’ve got the same problem. Using my old card 8800GT I had about 90FPS than i had temporally Radeon 1500 (something about it xd) and i had 20FPS, then I bought HD5770 and it stayed with 20FPS (sometimes less)
But whatever i do, my fps isn’t changing. For example, when i have resolution 800x600, AAx0, the lowest texture details i’ve got the same FPS rate when i have resolution 1280x1024, AAx16, the best texture details.

Also, when i run Gmod in -dxlevel80 even in -dxlevel70 my fps isn’t changing.

I’ve got:
OS: Windows 7 Proffesional
Processor: Intel core 2 quad Q9400 2.66Ghz
Memory: 4GB Ram
Video: Radeon HD5770 (Vapor-x model, 1GB)
Power supply: Chieftec CFT-650-14CS
I’m sorry for many mistakes but i’m not native english speaker

I just really what to know what is causing this. I play multiplayer all the time, and the low fps does not help with building.

I thought buying a better graphics card would solve my low FPS, but it seems its not helping at all.

Try removing all your addons? I did that and get 60+ fps on any map.

Well the only addons I have are PHX and Wire. and I use those all the time.

Do my specs. look like they should be running gmod better?