I have a 1993 Taurus. The engine cuts out for no apparent reason.

The engine of my 1993 Ford Taurus sedan cuts out from time to time. This problem has arisen in the last four months. It occurs about once every three weeks, on average, usually after I have been driving between 15 and 20 minutes. This is not a cold weather problem. Air temperature is typically between 50 and 65 degrees. There is no warning before the engine stops. It does not run rough or sputter, and it makes no noise when it goes out. It just stop. It will start up again right away within 20 seconds of stopping and then runs fine for quite a while. My mechanic, who is generally very good, can't locate the problem. I don't want to run the risk of driving the car in traffic until this is fixed. The engine has cut out twice on freeway on-ramps and that's no place to lose power. Thanks very much for your help.