Have you tried to use a different usb cable for the computer? Your current cable may be damaged which could cause this problem. Have you tried plugging it in a different computer? The fact the ac charger works leads me to believe that the usb cable you have is faulty or possibly the usb ports on your computer. I would try a different usb cable first. I had to fix my usb cable a few weeks ago as I was having the same problem. Three out of four wires on my cable had to be resoldered to the micro usb connection. After fixing the cable I have no issues at all.