When this happens, will the engine idle smoothly at 700 rpm if you hold the brake with your left foot ,and hold your right foot on the gas pedal with steady pressure? There is a modulator on the transmission with a vacuum line going to it but it has vacuum at all times, not just when the transmission is in gear. You should check all the vacuum lines if you haven't done so. They are prone to rotting if they've never been changed. Check the how-to guides for the location of all the vacuum lines.