How many things do you have plugged into your computer? A quick google search turns up several people having this problem, and a quick skim through a few threads shows that most people have a ton of crap plugged into usb ports. If you have too many devices plugged in and drawing power then I suggest you unplug some of them and see if the problem persists.
Or you can get a small powered usb hub of you do have a lot of usb devices plugged in and don't want to unplug them. Powered usb hubs have there own ac adapter so they don't use the computer for power. Just data.