We replaced the Torque Converter on my 2008 Ford Explorer. My mechanic said that replacing it should take care of the shudddering between 45-50 Mph. Well it has not taken care of it and now the engine races when I start to take off after stopping at red light etc. HELP, I am getting very frustrated with my mechanic

Tennisshoes, Thank you for getting back to me. I took the car back to my mechanic. He checked the fluid level and it showed full. Still does the same thing. The other day I backed in a driveway that is pretty steep to unload some boxes. The car sat for about an hour and when I got in to leave and put it in drive the engine just raced and would not move, so I put in 3rd and it lurched forward a bit and the same, so I had to put in 1st to pull the car out of the driveway. That is when I took it in and he checked the fluid and it showed full. The car ran so much better before he put in the new torque converter. Do I have to take it back to him. The car is still under warranty until the first of the year. Could the new torque converter be bad?