My son just picked up a 1989 Taurus w/low.

My son just picked up a 1989 Taurus w/low original miles. Very little drive time owned by disabled person. At times it runs fine, and times it will suddenly idle very rough and fight to stall. And sometime stall especially at stop lights or when driving slowly thru traffic or thru a turn after stopping. And then sometimes not re-starting and will have to sit for up to 30 minutes before it will restart and then may run fine or continue to run very rough. What thoughts do you have to trouble shoot. Mechanic hasn't found the right fix yet. Changed a vacuum controller, cleaned the throttle plate, adjusted the idle, etc.