If it's not the connection at the next receptacle as enblethen suggests (which I agree is the most likely culprit), then there's a bad connection in between the two receptacles somewhere.
You should be able to tell if the trouble is in the hot wire or the neutral wire by comparing the voltages to the ground terminal. If the hot to ground voltage is 121V and the hot to neutral voltage is 104V, the neutral is the problem wire. If the hot to neutral and hot to ground voltages are both 104V, then the hot wire is the culprit. If something else is observed, you may also have a bad connection in the ground somewhere or some other problem.