I have an 87 GT and have had surging idle problems at startup but it would stop after about 5 min. I cleaned the maf, throttle body, bought a new idle air control and tps but that didn't help much. I just finished rebuilding my engine a couple of weeks ago and now the surging idle has stopped so your problem could be carbon buildup. You may want to look into an engine treatment additive to see if that helps otherwise follow the steps in the maintenance section on idle problems. Hope this helps.