Stalled out

2000 bonneville ssei, 148000 mi. Car runs fine, good idle good power ect. But the other day, son, driving home, |@65mph, let off accelerator when going down long grade, motor died. Coasted to stop, stated right up, drive a few miles and same thing, let off gas pedal, die. I drove this morn, drove fine, made 1 stop and at next stop it died. Started up and drove on to work. Drove home fine. Any clues?