auto transmission problem

[snapgarrett]

I'm just curious if anyone has any idea what's going on with my car. When i start driving for the day or when my car sits for a while my transmission will act up. It will shift fine from 1st to 2nd to 3rd but it will stay in 3rd for a while then shift into 4th. Once it has shifted into 4th, the transmission will act normally for the rest of the time shifting up and down with no problems but its the staying in 3rd gear for a while that I would like to know about. These problems are occurring at highway speeds so I will be in 3rd going 60-70 trying to keep up with traffic and the engine is revving at a higher rpm than it should. I've had the car since 2007 and it has never done this before.