Is it a USB modem? If it is a combined usb/ethernet modem then I'd suggest you go the ethernet. If you don't have a ethernet card they're not very expensive nowadays. Many USB modems draw their power from the computer & also utilise your computer's processor. Thus, any major utilisations of your processor can mean inconsistent results from your modem.

See: http://forums.whirlpool.net.au/forum...cfm?t=86201#r5 & look for section called
3.4 USB-based modems/routers