I have a 2003 Ford Taurus 3.0L V6 Engine, when I have my car in idle it runs rough and the RPM's stay at 700, when I put it into drive it hesitates, if I put the a/c on the RPM's vary between 300-700 and then stalls out. I had routine maintance done recently, (oil change, air filter replaced,and tires rotated).The backpressure sensor was replaced,less than a year ago I had a tune up done.
Most idle problems are due to the throttle body or idle air control, which is located in the throttle body. I would recommend a good throttle body cleaning by your mechanic and possible replacement of the IAC if it tests bad.
I have also heard of vacuum issues and even spark plug boots improperly installed causing misfires when water get's into the spark plug area. These should be checked if your mechanic checks out the throttle body and finds no issues.