Strange FPS drop

Hi, i need some help

2 weeks ago my computer didn't start, actualy it did start but my screen, keyboard and mouse didnt start and the fan on my screen card made a wierd sound
So i hit the on and off button ~30 times, didnt help so i "kick started" my comp, and yeah! it fianly started properly
It started properly until windows began to load, a blue screen popd up.
I reboted 7 before i could enter windows and play gw
My FPS i usualy 60, but after 2 mins of playing it droped down to 40-30 and after an hour it went down to 20-10
And when i exit gw windows is verry laggish.
The lag disappears if i turn my comp on and off ,and the lag gets back after 2mins of playing after i restarted.

I have re-instald windows 2 times and the fps drop still wont disappear.
I have all drivers and stuff instaled

Here is my comp btw :
AMD Athlon 64 X2 4200+
ABIT KN8-SLI,nForce4 SLI, Socket-939
Corsair Value S PC3200 DDR-DIMM 512MB
Corsair Value S PC3200 DDR-DIMM 512MB
Gainward GeForce 7900GTX
Samsung SpinPoint P120 200GB

Thx for all help

Check for dust in your computer's fans especially your GPU's fan. Looks like your video card is overheating or is underpowered.

it might be a heating problem as someone here said

but cleaning a fan who is already working only increases its rpm a bit

so i suggest open the computer take a "BIG" fan and point it to the pc- that way he will be cold enough. even though it creates small tornados within the pc because of the small fans the big fan need to be big enough to overcome those small tornados and simply "blow the pc off"

now after doing this play as before - if you suffer the same problems than u can be sure that the heat is no problem..

if you have air-condition in your room and it is good enough u can just freeze to death while playing in order to check the pc (it worked from me though i wont recommend it at all).

Did you put your computer together yourself by adding bits to it? Or did you buy it ready made?

I ask as you list component by component, if you've been adding stuff lately make sure your power converter box (I only know it as 'Alimentation' in french) has enough wattage to cater to that 7900 graphic card.

Also NEVER NEVER NEVER kick your computer - dont come crying here if you've broken it.

I agree with BB, my first thought was the power supply. How much wattage is it(you dont actually get what it says on the box), and what brand is it?