Performance(gaming) Dips for a long period of time .Why?

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Hello
I have been having problems with my computer recently with its fluctuating performance in most of my games and including the benchmark i provided above.
Some games I play;
GW2 - it goes to about 18-53 fps but when its fluctuating it dives t about 5-12 fps..
Planetside 2- 18-25 fps but dips to 3-8 during fluctuations

These performance fluctuation last for days or even weeks and my computer only goes back to optimum performance randomly. I've been taking note of every driver update or installation i do...But even without changing anything these performance glitches happen.I would want to know if its my video card or my other hardware components who is responsible for this. IVe ran test like prime 95 and mem test for days and they show no error and when using my computer for everything other than gaming, it doesnt show performance loss at anypoint like slow loading etc. Ive also tried Downclocking my VC and performance just corresponded with the changes. My primary suspects are my VC and my generic PSU(< i really hope its this one since i had no problems with my 400 watt seasonic before and had to buy emergency 600w generic psu)
specs:
e5300 @ 2.85 ghz
9500 gt mem clock 440
3gb ram ddr2
600w generic PSU
1280x 1024

Hope someone could tell me which hardware is responsible..
All help would be appreciated.. Thank you very much in advance