Stalls, red triangle, check engine, coming and going

A few weeks ago I got out of the car, leaving it running, to pitch the garbage in the dumpster. When I got back in and put the car in drive, it would not go. I put it in reverse. It would not go. I turned it off and restarted, and managed to get it in reverse but it wouldn't go forward. I restarted it again, and it worked like nothing had happened. This lasted three weeks. Shortly after, the check engine light appeared. Auto zone checked the codes and said nothing to worry about. A used prius dealer told me don't worry, it is just something that makes your heater hotter in the winter. (I'm in FL and it isn't winter.) Then three weeks passed with nothing else happening, except the check engine light seems to appear and disappear from time to time. Suddenly, turning into a driveway, the car stalls. I restart it and it stalls again. I restart and it makes it up the drive, but the red triangle and check engine light are on. The next day, I go to start it and the lights are back off and the car is running fine. It's not out of oil. The little battery is still good. It sounds like it usually sounds and other than these incidents, drives normally. What can possibly be going on?