Sometimes stalls on 90+ days

My 2003 3.0L sometimes stalls when trying to start on 90÷ days. If you manage to get going and take foot off gas tach immediately drops to or near zero. Doesn't matter how fast you are going. Driving for 15 or 20 minutes will make condition go away. Mechanic cannot see a problem with engine. Of course this never happens for mechanic to see.