I have a Geforce 5500FX 256 MB and when im playing battlefield online (gamespy) it will randomly (but normally when i enter games) make a windows notification sound and turn the game off -no error sign, no nothing just back to windows.
Ya if it's only doing it while on punkbuster then I'd imaging that it's got somthing to do with connectivity. As Praetor said it could be a firewall and check GS settings and such.
most people say they get a error message when it kicks them out but while bf1942 is loading to connect to the game i get the windows notification sound and the game just shuts down and returns to windows.