Try changing usb ports. If possible plug it into a usb powered hub or a pci sub card. I had the same problem with my mouse. Would just quit working. Unplub and replug and its back. Eversince I put it in a usb 2.0 pci card it hasnt dropped off once.
It could just be a driver issure or maybe the usb is losing power or something. I quit worrying about it since it stopped messing up.