USB Device Error

Running an Athlon 1.2 Ghz with Windows 98SE and 512 MB RAM. Using an external USB modem (Zoom 56K) to connect to the internet. When using modem, the USB ports will shut off at random time intervals. Either while dialing, or after being connected for a few minutes. Both USB ports shut off, and this happens regardless of which port it plugs into. Other USB devices do not cause this problem. Suggestions?