DSL modem hooked to usb port

I have hooked a 2wire brand dsl modem to a emachine
computer using the usb port. Apparently trying to install
an ethernet card does not work properly. The emachine does
not recognize the card. 2wire suggested I hook up the
modem to the usb port. I get good connection to the line
outside but the cable from the modem to the computer keeps
getting interupted intermittently. Some times the
connecton light will stay on for hours others for minutes.
I have changed the usb cable and tried another usb port on
the front of the computer. Same problem.

Any help will be appreciated.