Q: Car overheated while idling. What is wrong

A few weeks ago, my husband was idling in a parking lot. It was a hot night and he said that the car overheated. We could not reproduce the problem, but since then I have been watching the temperature gauge. Today, I was in a drive thru and while waiting in line, the temp sensor went up and alarmed. I smelled antifreeze, but air-conditioning still worked. I turned the car off and back on when I needed to move forward. I kept turning the car off and on until I could get out of the line. Once I was able to leave the drive thru, I noticed that the sensor went down to normal operating levels as I drove. When I parked in my driveway (less than 2 miles from drive thru) the level was below the half level mark. While idle in the driveway, it went up a notch to the midline (where it normally operates). I then turned it off.
My car has an automatic transmission.