What Motherboard Procy and other stuff u r using ??
It happend with us once with few of our 915 GAV/GEV based boards, we found that, to save Power Computer was turning off the USB devices, for a temp solution, what we did is,
Go to
Device Manager, navigate to
USB Root HUB or devices, Right Click on to go to [b]Properties[b],
Navigate to TAB power Management and turn off the Option,
Allow computer to turn of power to this device to save power
and for Permanent solution we changed all the SMPS of the samne config running systems to a Higer Capacity PSU