1987 Oldsmobile 88 Mechanical or Electrical. Service engin

After driving about 30 minutes the service engine soon light comes on, then 5-10 seconds later the amp light and the car dies. Over the next two days it does it again twice. We sit on the side of the road for 20 minutes and then it starts up and is fine. We had it to the mechanic and he could not get the car to do it. Two days later we are driving and the service engine soon light comes on again. We turn off the heater and radio. Continue driving and the amp light does not come on all the way to our destination.