First off, there is no "common". It is the neutral. And the voltage is not on the ground as your title states, it is on the neutral.
Your symptoms are classic of an open neutral on that circuit. You are reading voltage through a load connected downstream of the device you are checking.
Turn off the circuit and check EVERY device on that circuit. If the wires are simply stuck in the back of the receptacles remove them and place them on the side screws by making the proper "J" hook on the end. The quickwire connections are notorious for failure and can create a situation just like yours.