2007 Ford Focus stalls out while driving randomly, now won't turn back on?

The car is a 2007 Ford Focus, automatic drive, and sometimes randomly it stalls out while we're driving. Usually at stop lights or signs, and the car has to be turned off and back on again, but it runs "normal" again after that. It's not everyday, but sometimes 2-3 times a day instead of just once. Today my boyfriend drove it home and it died and wouldn't start up again. What could be wrong and is there anything one of us could do?
** sometimes when we're parked or stopped at signs it makes a revving noise on and off. It sounds like it's sort of flickering, but we had a mechanic look at it a few days ago who said everything seemed to be fine other than we're due for an oil change. (Still not empty though.)

Read more: 2007 Ford Focus stalls out while driving randomly, now won't turn back on?