Via chipset on the motherboard could be the culprit. I copied this fom 'somewhere':
There is a power management problem with some motherboards, including those using the Via KT7 chipset (which is found on several models of motherboard). This causes the USB ports to loose power at apparently random intervals. The symptoms would be that the Broadband ADSL will usually work fine for a short time and then the modem would just switch itself off or lose connection. After a reboot of the computer it will be OK again for a few minutes.
If you experience this problem we would recommend that you seek advice from your computer supplier or purchase an externally powered USB Hub.