I recently built a new PC (HD4850 video, core 2 3.0 Gigabyte p35 mobo, 4 gig ram).
I can play single player all day long without a problem. I played online for about 4 hours also without a problem until a hacker (I use that term loosely since 99.9% of arma hackers are no talent dweebs who googled their hacks) caused all kinds of mayhem on the server I was on (battle eye completely ignored him btw).
Since then If I try to play online I will quickly get a windows error program needs to close prompt. Arma remains open and I can maniuplate the menus ect but my connection to whatever server I was on is severed and ArmA no longer has network connectivity.
I tried uninstalling and reinstalling both ArmA and Battleeye but the problem remains.
I can still play single player for hours without a hitch.