Could be a number of things...stuff I would try...
- a different cable is the first thing that comes to mind
- check the socket in the unit isn't loose (can be a solder joint misbehaving at that point)
- a reboot of the machine prior to hookup may also help
- if you've got batteries in there, take 'em out and allow the unit to be powered exclusively from the USB cable
- conversely, power the device from batteries only and make sure the device really is working
- Barter-esque swearing may help relieve some stress on you but maybe not the device
Greetz
S.