I had the same problem on my 83. what happened on mine was over time the cable and the throttle linkage stretched and wore. Things weren't real tight anymore. Find the cable that comes from your cruise control diapram that runs to your throttle linkage...make sure there's no slack in it. If there is, adjust the cable. Make sure theres no lsack but don't over adjust it or the throttle linkage won't return to its proper resting position. Whats happening while your driving is the car gives it a little gas, lets off...and when it goes to give it gas again...it doesn't do anything so the diaphram pulls harder and then you accelerate...slow down, accelerate...and so on. Let us know if you get the problem fixed.