Why does my HD5770 run at x2?

[minikahona]

Hey everybody! I have this problem for a week now, and I dont know what should I do. My PC is 7 years old, almost 8, so I don't know which part isn't working.
It started about a year ago, when the windows update downloaded the latest AMD driver, but you know, it isn't compatable with my card. After I uninstalled it and installed the correct one, the GPU-Z showed that the card is only running @x4 insted of x16. I reinstalled windows, and it was x16 again. But about a month ago it started again, but now without the wrong driver. From one day to another it went down to x4 again. I tried everything (cleaning the whole machine) but nothing worked. But because the performance didn't changed, I tought it's just a bug. Now a week ago it went down to x2, and now I can sense som performace drop. For example in Guild Wars 2 the FPS went from 25 to 15 in average. But it's weird, because in some games I can't sense any real difference. Maybe it's slower, and I just can't remember how it was before, nowadays i only play GW2.
My friend came over, and we tried his card (a HD6670, it doesn't need extra power from the PSU), and it was at x8 with 4GB of RAM. After I put an 8GB module into my computer (by accident, I tought it was a 4GB module), it went up to x16 with my friends card, but it stayed x2 with mine. I put my card into his PC, and it was only x4. The problem is that his PSU is weaker than mine, so i don't know if my PSU is dieing or my mobo or my graphich card.
So I don't know which part of my PC causing the problem.

My config is:
Mobo: Asus M4A785TD-V EVO U3S6 (with latest bios)
RAM: 2X 1GB KINGMAX 1333 + 1X 2GB KINGMAX 1333 (I had +1 2GB Kingmax, but it died a week ago when I took apart my PC)
CPU: AMD Phenom II 955 Black Edition 3.2GHz
Graphics: Sapphire HD5770 Vapor-X 1GB GDDR5
SSD: Samsung 840 Evo 250GB (connected to the U3S6 card)
HDD: TOSHIBA DT01ACA300 3 TB, 7200 RPM, SATA-III
PSU: GIGABYTE Superb 550P
5X 12CM led fan
Windows 7 x64 / Windows 10 x64 (I tried both of them)

What do you think, which part of my PC causing the problem?

PS.: Sorry if I made some mistakes, I don't speak engilsh very vell.

 

[dontlistentome]

Since your card in his system was only x4, I think it's your card. The card is not very demanding on power so if your power supply was dying I'd expect you to have problems like the system shutting down or randomly rebooting. If it was the motherboard then his card would have had some issue.