It sounds to me like you are consuming too much power form the USB, which is limited to 500mA. Easiest fix is not to use the USB port to power the board.
Gerald
If you must be connected with USB and need more then the 500ma a standard USB connector can deliver
you can use a cable like this that comes with some USB hard drives
http://www.usbfever.com/index_eproduct_view.php?products_id=167 ( not an endorsement search for a better one/price )
it will allow you about 1 amp from your PC