transmission

Another transmission problem. I recently changed my transmission, its a 4t65e for a 3.8 ltr. Now it feels like it is slipping, when i get to about 40 mph the rpm drop and the car starts to shudder or miss, when i let up off the accelerator the rpms go up and it drives fine. Can someone tell me what this may be. There are no codes

Kinda sounds like torque convertor lock shudder, about the right speed and condition for it. If that's one that you can unplug the lockup, you could find out real quick.

I will be taking the car to the dealer and see if they come with something like that. I just want to have some kind of an idea so i won't get ripped off... How do i unplug the torque convertor lock up?