This is very difficult to understand. I'll give it a try.
When an Automatic trans with the stock Valvebody is converted to Manual Shift the Valvebody is modified or replaced with a Manual Valvebody. If it has an electric Transbrake it will have a Solenoid to activate the Brake.
It sounds like the Detent/Downshift cable is adjusted too tight. It should downshift to 2nd gear from 3rd at around 3/4 to Full throttle.
The problem with it getting stuck in 2nd may be the Detent valve in the valvebody sticking/binding or other shift valves in the valvebody sticking/binding.
Called Detent cable, Kickdown cable, Passing gear cable.