Shifts into neutral while driving

It was really hot and I was driving up hill for four or five miles when I started downhill and stopped at a stop sign the car went into neutral and would not move. I put it in park and turned off the engine. After a few minutes I started the car put it in gear and it ran fine. This happened several more times then has not happened in two or three weeks. But I am afraid to drive it anywhere. Any ideas? A new transmission was put in at 60,000 miles.