a wireless card may be the problem, some cards will drop connection if you try to use too much bandwith, that is probaly why your connection is going down all together. What signal are you picking up? are you on a GSM or CDMA carrier and by what signal i mean, RevA/EVDO/1X (those are the CDMA versions, that is all i know)

RevA you shouldn't have a problem, it's going to be like a between dsl and cable
EVDO you really shouldn't have a problem either, it will be a little laggy but it should work
1X, forget it, that is like dial up, you'll be lucky to run 2