99 Maxima electrical/engine dies

We have a 1999 manual Maxima. Several times while driving down the road, all electrical systems have died causing the engine to die also. It will usually crank back up and run fine until the next time it happenes (about 3-5 times a week). Last time it happened the engine cranked back up but the electrical systems were flickering on and off. Our mechanic has had it for two days and cannot diagnose the problems because the car hasn't done this since he's had it. Anyone have any idea what this is? Thanks