Transmission Slip Out Of Gear problems of the 2000 Dodge Avenger

Sensor in transmission quit working. The first time I was driving on the freeway and it completely lost speed. I exited and came to a complete stop. After that, it would not shift out of first gear. It was repaired by a dealership under warranty. That was at around 20000 miles. Today, at around 42000 miles, I started the vehicle and was going about 20 miles an hour and could feel the transmission beginning to slip. I started increasing speed, and immediately noticed that the rpm's were just continuing to go up. I have sent the car to a dealership to be repaired for the second time under an extended warranty.