2000 Pontiac Grand Prix Shifting problems

All of the sudden, my car was having a hard time starting from drive. I have to put the car in 2nd gear and drive it like a stick shift until it gets going. It is fine once it is up to speed. I took it to a transmission shop and they said it came back with 8 codes. So I bought a used transmission and had my friend put it in and now it is doind the exact same thing! The computer at an auto parts store came back with a code for shift solenoids. I took it to another tranny shop and they said it needed to be rebuit for $1500.00. I found another tranny shop to look at it and they said it could be something electrical. The mentioned the computer (?) Or the Torque Converter (?) Someone else told me it could be the Catalytic Converter (?) I don't know anything about cars and feel like everyone is trying to screw me over. I just put a new motor in the car less than a month ago. I really have no idea what the mechanic is talking about and hope you can give me an explanation on what could possibly be wrong.