FPS drop after 5min of gaming?

Hello,

I have recently upgraded my motherboard an gpu to an Abit Kn8-SLI and BFG 7600gt OC vid card. However, when I first start playing a game (mostly WoW, HL2 or DoD:S) the framerates are great. 60 fps+ but after about 5 min of play it drastically drops to somewhere around 20-30 fps. If I restart my system they seem to be great fps again but the same thing happens. I have the newest nVidia drivers, directx 9.0c, I've tried reformatting, but nothing seems to help this problem. I've also tried stopping non-critical processes running on the computer. Before I had an ATI 9800 All In Wonder with no issues like this. Any thoughts on this issue would very helpful. Thanks in advance!

System:
Abit KN8-SLI
BFG 7600gt OC
Athlon 64 3500+
1gb RAM
Soundblaster Audigy 2ZS

usually its due to drivers or possibly OVERHEAT. Is you case properly removing heat from your system?
Did you remove the stock heatsink and replaced it with an aftermarket one?
try your Nvidia settings, there should be an option where you can rise the meter so that your card wont throttle down in certain temps.

What kind of psu do you have?

Sorry for not having a quick response. I posted this last night from work and I work the graveyard shift, so I just woke up. OK, as for the overheating issue, I did a little test. When FPS dropped I checked the GPU and CPU temp. GPU was hovering right around 55C which was only about 2 degrees higher than when computer is at idle. CPU temp was about 50C so I dont think overheating is the issue. I have adaquete cooling inside the case (3 fans) and I have the factory fans on the CPU and GPU. It doesnt seem to matter if I'm inside our outside a building in the game, it just does it. And drivers, I have what I think is the latest DirectX drivers, and I have the newest nVidia drivers. The only thing I havent tried was flashing the motherboard bios to the newest one.