I had my U3 plugged into a powered USB 3.0 hub plugged into a 2.0 port on my computer, it didn't like this, lots of LIBUSB errors.  Now running in a powered 2.0 hub, seems to be happy for ~24 hours (knock on wood).  Using a new power cable, too.  This one is a molex 4-pin to barrel plug and has thicker wires than the pci-e to barrel plug I was using (2.5mm vs ~2.1mm).