1991 honda civic dx - engine cuts off while driving

I have a 1991 honda civic dx.  Worked great until last week when i was stopped at a red light and the engine just cut out.  Restarted OK.  Then i was cruising down the freeway and the engine suddenly cut out again.  I pulled over into the shoulder and it started fine.  Now, i can't make a 5 minute trip to the store without it cutting out, and it takes a few tries to start.  My mechanic cannot locate the cause.  Anyone?

If you have never changed the main fuel relay do so now. Hondas of that vintage are notorious for a problem with this relay.