Possibly try a different charger? Is this the USB cord which came with the phone? Not all USB cords are created equally. Not all chargers are created equally. If it does the same with another charger(preferably with the original USB cord), is the phone still under warranty?
As Rc indicated some USB's can have issues I have't seen any troubles in the one's that I currently own but I have more than one that I can use Plus I have the old Traditional OEM charger's that Motorola currently sell's but's not included in the box with the phone) The one you see below is one that I have and is usually found with an accessory..
If the phone is not passed the 14 day return, box it up with it's papers and return it to the store and have a Rep check it out with another charger if it works ok and you want to keep it Go a head) But if you'd rather exchange for another one see if the Rep will let you do it. Good Luck on your Decision.. b33