Massive frame drops when playing MW2... help?

So, I've been getting massive frame-drops when playing online, like way below 20 FPS. For the most part, gameplay is 90+FPS, even up in 115FPS range at times. What puzzles me is I can't find a single reason for it either, at least on my end. I have a 4.2GHz 2500k, 8GB of DDR3 1866 RAM, and a Radeon 5830. I know for a fact my rig can handle this game on high settings fluently, but even dialed down the settings to low  and disabled AA and vsync, but I still get frame-drops. I also under-clocked the GPU core (thinking it was throttling because of heat) to 775MHz, and even FurMark can't push the core past 67c on a 15min 1080p Burn-in. Any tips/input on finding the cause?

Would've posted in Steam Forums, but it's a headache. Apparently I have to have a seperate account for Steam Forums and I'm still awaiting to be approved by moderators, dispite clicking the activation link. *-_-