I was gonna say check your batterry connections......Because this is the exact same problem I had a month or two ago. And it just turned out to be very dirty terminals and a bad batterry connection.

It was the gound to the engine bay from the negative terminal wire. When i re-did my alt and ground cables, the connectors proved to be not as good as the original hardware.
Man, you have no idea how happy i was earlier today. My car has been sooo reliable for a while now and then this happened. But is was fixed really fast and easy so this kinda dosen't count. (especially cause it was my fault ) Thanks guys!