Engine Shut Down on my '87 300E

Last week a friend and I took a drive out to Roslyn Harbor Long Island, about 35 miles away. On the way back before coming to a complete stop at a traffic light, all my idiot lights came on and I realized the engine had shut off. I shifted to neutral and turned the key, the car restarted with no problem and we continued on our way, back to the city. The next day, I gassed up at a mobil w/ the usual 93 octane, came to a halt at a stop sign and the engine quit again (no sputtering or knocking)just a complete shutoff without warning. I again restarted (this time after a couple of tries) and drove across the Manhattan Bridge. I continued on a couple of blocks and again the engine shutoff. (this time it would restart only after about a 5 minute rest)I then drove another couple of blocks and it shutoff again. This time I noticed a little condensation at the bottom of the windshield so I added a little water. I drove all over Manhattan until the engine was hot and the problem seemed to be corrected. The next day the car was very difficult to start, but when it did it purred like a kitten (as always) and then promptly shutoff, without restarting at all.
would anyone know if this is electrical or mechanical or what could cause this problem....
Thanks jsails46