I have a 2002 Silverado work truck with a 5.3 in it. I am having problems while driving on the highway where everything is fine until i reach about 60mph. NORMALLY, when I am around 70mph the rpms were at about 1800-1900. I had one week where I was putting more miles on my truck then I ever have and after that one week my transmission hasn't been the same since. But back to it, now when I try to reach 70mph it seems like the truck is struggling to get there. Also the rpms will not stay still. They jump from around 1900-2600. We went to a local junk yard who parts out wrecked vehicles and got a used transmission from there and switched it out but it is still doing the same thing. The only code I had was a knock sensor code of p0332 but I changed those out a while back. So we went to a tune guy who did an override on the knock sensor code and added pressure to the transmission to help with shifting, whatever that means, but the RPMs are still not holding steady around 60 mph. After doing some research my thoughts are either the torque converter clutch is out, or the computer is used to the old transmission and is running the same process with the new transmission too. Has anyone else had this problem before and how did they fix it?

p.s sorry for the long explanation, I just want all factors taken into consideration.