car died without warning

I was coming off I-77 turning right to merge with traffic from the off-ramp when the car just died. I had been traveling 70 mph on a crowded highway that has frequent tractor trailer accidents due to the grade. If this had happened on I-77, then I might not be here to tell the tale. Since that time, the warning lights (seat belt and battery) intermittently flicker on and off. I have noticed when the battery light comes on that the fan slows. When the battery light goes off, the fan returns to normal speed and output. I have taken the car to two different auto part places who hooked it up to their machines and declared that "no codes were being thrown". I have also taken it to the dealership, who said it needed a computer update. They did this for no charge, but declared that everything else looked great. (The lights are flashing with more regularity now.) The battery and alternator both tested fine. If anybody has suggestions, please let me know. I am beyond frustrated and have no car know-how.