1988 Ford Mustang idle

My son just drove from school in canada to durango colorado in his 1988 mustang. I went out to look at his car for the first time. What I noticed was that his car had idle surge it would go from 2000 rpm to 14000 but would not go any lower unless hard shifted into gear.I tried to adjust his idle screw but it was already adjusted down. I would appreciate any information you could share. Thanks