Only multilayer performance issue

[ezerb 10]

when I load up in a game online, it runs very smoothly but over time I start to loose frames or something because everything gets all laggy. This happens from when I start to 10min later, so is it Arma's problem or mine because if it runs smooth it should stay that way. my specs are

duel core 2.6ghz intel

Nvidia 460GT

4gigs of ram

PS. this problem happens in every game I join