Engine Performance problem
2001 Ford Taurus 6 cyl Two Wheel Drive Automatic 130000 miles
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The car idles rough and hesitates when stopped at a light when the air conditioner is on. If I put the car in park the idle is fine. I have had it to my mechanic several times and he has done different things to it. The computer says everything is OK.

Check for vacuum leaks
check idle air control valve

first i would try cleaning out the throttle body and checking the IAC module.

then i would check for vaccum leaks. do this by using a can of carb cleaner and spray around the vaccum lines and air intake hose and manifold.