usbman.com noted motherboard drivers (my recurring reply?) and then dive into usbman and his specific tips for each VIA and other makers.

The last item is to toggle the power saving setting of the USB device or hub. Noted at usbman.com

-> Frankly I found USB dialup modems to never approach the stability of RS232 connected modems. Never could understand why this was so except the makers must be cutting corners.

Bob