Owners/service Manual problems of the 2006 Mercedes Benz E350

I was driving my vehicle when suddenly the vehicle began to stop and the transmission appeared to be frozen. As I put my foot on the accerator the car would increase speed then it would again slow down. When I pulled over I was able to switch off the ignition and restart the engine and resume driving. Once I got to my destination I was able to put the car in reverse with no problem. When I got ready to travel back home I began to experience the same problem. However, I was now able to only drive the car using manual shift or drive in 1st or 2nd gear at 20 to 30 mph. I had the car evaluated by mercedes in atlanta and was told there was some problem with some type of transmission pump and that I will need a new transmission. My car has been maintained by mercedes since I purchased this vehicle. I have never had any trouble with previous cars that I have driven for 10 plus years. Please investigate this matter for I was told this is not unusual for this vehicle. A new transmission will cost me $6500. I can buy another car for that price. I did have an extended warranty but it has now expired.