Transmission slips into low gear

When the outside temperature is over eighty degrees and car is driven half an hour or more in stop and go traffic, instead of shifting into higher gear, it shifts back to low gear. Will stay there until I stop and turn off the engine. After engine off for about fifteen minutes and restarted transmission shifts fine, until the next time under above circumstances. It has been doing this for two summers now a total of maybe ten times it has never done in in the winter even on long trips with driving in traffic and most summer driving is limited to to and from work (ten miles one way).

Dave