Left for dead 2 issues?

ive finnaly got a bran new graphics card (nivida 9800 gt) and before that i could play left for dead 2 just relly laggy.... and also play onlye ... also laggy...but i install my graphics caard and install the latest driver and the game seems fine but lags a littel..(could fix by turining down the graphics) but when i play mutiplayer .. it dosent load it says microsoft runtime error has caused to shut down this app. click to contuinue..... the game works on single player but not on mutiplayer .. pleese help!