1990 Buick Lesabre Engine Stalls or Cuts Out

I am the original owner and car is well maintained with all needed maintenance done by a dealer. It is not a beat up car. Problem has happened 3 times in last few weeks. After sitting for a few hours at work or at a store, it starts and dies immediately. Willl not stay on unless I keep my foot on the gas pedal for about 5 minutes. Afterwards, it is fine. One time it died while coasting at about 30 MPH but started again and went on with no problems. Starts fine in the cold weather overnights. Took it to a independent service center as well as a dealer but they could not find any codes and could not duplicate problem.