My mechanic just completed swapping out the original tranny with a used tranny (both marked 7G on the bell housing) and rebuilt torque converter. Now I'm having new issues.

I have no problem going into any of the gears, but now it doesn't have any power when I take off from a dead stop, and climbing hills, but runs fine on the fwy past 70 MPH. It doesn't shift right from a dead stop and when I do get it moving the rpms jump to 5k before it shifts or I have to let go of the gas before it shifts.

It seems like it starts in the higher gear when I take off from a dead stop since I can pump the gas pedal, but engine rpms don't go up.

Anyone have any ideas on what this symptom could be? I don't want to take it back to the "backyard" mechanic who worked on it since he doesn't know much about automatic transmissions.