Very weird graphics problem/mouse related

Hey guys, i have a system with 2 GB 667 MHZ Kingston Hyper X Ram, Gigabyte GA-P43-S3L motherboard, Intel Core 2 Duo e6600, Asus EN8800 GT (revision 2), Sound Blaster Audigy Value and Samsung 19 inc monitor (400 W AOpen PSU).

I have an problem that i don't know when exactly it started (but it's probably after i installed the Core 2 Duo OR a power failure. I don't know which one). The problem is, when i'm playing games mouse movement is not very smooth and looks like it's skipping frames (you cannot notice it in the dark but in the areas with dim light in games it's noticeable and very annoying). I tried everything - formatting, reformatting, trying lots of chipset and videocard drivers, messing with BIOS options, messing with DXdiag options: nothing helps.

But there are some things improve the situation:
-I was using the mouse with an PS/2 adapter. I removed the adapter and connected it by an USB port. Smoothnes of mouse increased dramatically for some reason (but still it's not very smooth).

-When i set my monitor's HZ from 60 to 75 hz, it happens on 75 fps and when it drops to 60 fps the game becomes very choppy(that's supposed to be smooth). When i set it to 60 HZ it's MUCH smoother on 60 FPS.

-AS I SEE IT (i'm not %100 sure) it doesn't happen on DX10. Probably only DX9 drivers.

The things which are NOT responsible for the situation:

-The motherboard. When the problem occured i was using a DX975XBX. Now i changed it to Gigabyte and it still happens. Nothing to do with this.

-The sound card. I removed the drivers and the sound card altogether and still happens.

-Memory. I used different memories and still happens.

The things which can be responsible for this:

-400W PSU
-Asus EN8800 GT
-C2D 8800 GT

I just wanna know which one causes the problem before i make another upgrade. Any help is appreciated as i'm struggling with this problem and this is driving me nuts. Thanks very much.