random vibration on highway rpm varing 500 rpm

Most people that have this issue say the rpm gauge (tachometer) is jumping up and down. Especially at idle. It is typically associated with a clogged idle air control valve or a throttle position sensor. Since the vibration comes at highways speeds, see if you notice an exact speed that makes it occur, or if there is a specific point in the gas pedal that makes it vibrate. See if your air conditioner changes the vibration, and see if the vibration occurs when you change lanes, go over a specific portion of the highway, slow down at high speed, etc...