Possible Asus P5N-E SLI issue

Hello, I seem to be experiencing a possible performance issue that appeared recently. When meeting specific graphical features in games and such, the frame rate drops drastically, a lot harder than I have ever experienced in my gaming life. The facts and tests i've checked are the following:

My computer:
Asus p5n-e sli mobo
Intel core2duo e6420 @ 3.20Ghz
2xGeIL ddr800 1G Dimm, @ 800Mhz
Asus 8600 GTS 256MB version
620W power supply
buch of hard drives
passive huge cpu cooler
fans on the side of the case

My bro's computer:
Abit kn9 ultra
amd athlonx2 4600+
2xGeIL ddr667 1G Dimm
Leadtek 7900gs
stock power supply
etc

I experienced a hardware fault recently, right before this problem appeared. My computer didnt start up, blaming the video card(1long and 3 short bios beeps), the gpu was sent for repairs and was returned, they said it didnt have any problems to fix on it, it was working fine. Tested it on several computers, didnt work on my bro's computer either(same thing, 1long, 3short beeps), but surprisingly every other system i tried it in accepted it without a doubt. Later I partially solved this issue by removing the SLI selector card(or whats it called) from my mobo, the performance issue started then.

So I started doing tests. the 2 computers compared are the ones above. They're relatively close in performance to each other, as they were for the last 1.5 years. 3dmark05 1.30 was used to test them, and the results were rather confusing.

The following settings were used on both computers:
- 1280x960
- 4 sample AA
- 16xAnisotropic filter

And the results were the following:

- My computer, gpu=8600gts

3DMark Score 5028 3DMarks
GT1 - Return To Proxycon 29.4 FPS
GT2 - Firefly Forest 12.5 FPS
GT3 - Canyon Flight 22.2 FPS
CPU Score 2661 CPUMarks
CPU Test 1 1.9 FPS
CPU Test 2 1.7 FPS
Fill Rate - Single-Texturing 2623.0 MTexels/s
Fill Rate - Multi-Texturing 10370.5 MTexels/s
Pixel Shader 155.6 FPS
Vertex Shader - Simple 20.2 MVertices/s
Vertex Shader - Complex 36.8 MVertices/s
8 Triangles 4.2 MTriangles/s
32 Triangles 12.7 MTriangles/s
128 Triangles 25.6 MTriangles/s
512 Triangles 34.3 MTriangles/s
2048 Triangles 37.3 MTriangles/s
32768 Triangles 38.2 MTriangles/s


- Bro's computer gpu=7900gs

3DMark Score 5028 3DMarks
GT1 - Return To Proxycon 23.7 FPS
GT2 - Firefly Forest 15.1 FPS
GT3 - Canyon Flight 25.8 FPS
CPU Score 10038 CPUMarks
CPU Test 1 6.9 FPS
CPU Test 2 7.1 FPS
Fill Rate - Single-Texturing 3444.2 MTexels/s
Fill Rate - Multi-Texturing 8105.4 MTexels/s
Pixel Shader 120.2 FPS
Vertex Shader - Simple 57.9 MVertices/s
Vertex Shader - Complex 41.4 MVertices/s
8 Triangles 2.2 MTriangles/s
32 Triangles 7.2 MTriangles/s
128 Triangles 22.9 MTriangles/s
512 Triangles 53.6 MTriangles/s
2048 Triangles 81.5 MTriangles/s
32768 Triangles 96.6 MTriangles/s


- My computer, gpu=7900gs
3DMark Score 4402 3DMarks
GT1 - Return To Proxycon 24.6 FPS
GT2 - Firefly Forest 11.0 FPS
GT3 - Canyon Flight 20.1 FPS
CPU Score 3315 CPUMarks
CPU Test 1 2.3 FPS
CPU Test 2 2.1 FPS
Fill Rate - Single-Texturing 3527.0 MTexels/s
Fill Rate - Multi-Texturing 8841.1 MTexels/s
Pixel Shader 129.7 FPS
Vertex Shader - Simple 19.9 MVertices/s
Vertex Shader - Complex 35.7 MVertices/s
8 Triangles 2.0 MTriangles/s
32 Triangles 6.3 MTriangles/s
128 Triangles 16.9 MTriangles/s
512 Triangles 29.2 MTriangles/s
2048 Triangles 35.4 MTriangles/s
32768 Triangles 37.1 MTriangles/s

- Bro's computer, gpu=8600gts

Boot failure, same as before:1long, 3 short beeps.

now im not sure if it means much, but there IS a difference in processing power if we take a closer look at the results i had with the 7900gs. My computer's stuck at ~38 MTriangles/s and my bro's comp can go all the way to allmost 100MTriangles/s, also this is the first time I saw an AMD beat a core2duo so hard. Vertex shader performance is under the line aswell... a 30MVertices/s isn't normal imo.

So in other words there are problems here, one may be the graphics card for not starting on specific systems(and only starting on mine if i remove the SLI selector card completely), the other may be the motherboard perhaps? frequency misunderstandings between pci-e and cpu or, honestly i dont know...
Anyone got any ideas what could be wrong? Drivers are all topped and whql(tried betas aswell, no change), bios flashed recently, and my windows was installed not long ago too.

Thanks for any replies, in advance.
Yours sincerely, Baranyi Andor