Transmission code P0740

Bought this vehicle two months ago with no codes active or stored. I drove the car for a week or two and noticed at high speeds the car would downshift hard when I let go of the gas. This does not happen every time in the two months I’ve owned the car it has only happened twice. One day I start the car up take it to the corner store and when it comes to a complete stop the engine stalls out. I turn the car back on and it works fine. A week later the same thing happens followed by the p0740 code. I took the vehicle to Aamco who told me the whole transmission needed to be replaced $2,500.00. I stuck a bottle of Lucas oil transmission fix in and the car stopped stalling. I did get the code again after clearing but no dramatic downshift or stall this time it just popped up at random one day. I took the car to a local transmission shop. He hooked his scan tool up and took it for a couple drives and found no issues. I wanted to believe the guy but there was a language barrier so I took it to another local transmission shop and the guy wouldn’t even do a diagnosis. Told me that from what I’m describing the torque converter is failing for sure and the converter needs to be replaced and transmission rebuilt $2,300.00. I guess my overall question is, could this be a simple TCC solenoid gone bad, electrical issue, oil pressure inside the transmission or are these guys right and the whole transmission needs to be rebuilt? It works fine now with the Lucas oil in but I drive this thing on the highway and the last thing I want is it stalling doing 75+mph. Thanks for the advice in advance.