Shut down

I was dring the other day and when I went to pass a car my car COMPLETLY shut down when it went to down shift. EVERYTHING was off for about 4 sec then it all went back on again as if nothing happened at all. The clock didnt even reset. The connections on the battery and alternator are good and it hasnt done it since but that was only 3 days ago. I was hoping that someone had an idea to the symptom/cure. Thanks.