I don't think it is your 3G connection. I have a MacBook Pro ( 2011 ) running Lion. My messages ( and iChat too ) will only stay connected for 5-10-15 minutes and then disconnect. I'm on really good networks at my work ( but going through a proxy ). I don't use other IM tools BUT I did try Adium and do not have the problem. Fairly convinced it is totally to do w/ messages/ichat.