does the whole machine lock up? I’d monitor your temps (cpu and gpu) to see what they’re doing. I imagine your video card is getting pretty warm.
did you try turning down some settings and trying it again? I believe the multiplayer is a little more intensive than single. Other than that, not really sure. I’ve never played it online… however I got a freeze here and there using my 8800 and every time the temps on the GPU core were around 75+ degrees.
You can crank the fan speed up in rivatuner to help combat that