Absolutely The same thing for me, I Can run the game on Ultra settings in single player,and get between 40 to 60 fps but as soon as I try playing in multiplayer... Doesn't matter if I set everything to low and turn off everything that I can I still get between 10-to 15 fps

AMD Quad Core 9950 2.61ghz
16gigs of Memory
3gig GTX 580