The key to his problem is mentioned in post 5... The cart rolls, and if he presses the brake pedal, it affects the response...
This indicates a Motor brake that is drawing too much current (impedance under 20 ohms), and a possible shorted Motor Brake FET. Error code in the controller should be reading Motor Brake OC Error and Motor Brake Driver On if the FET is shorted...
Remove the plug from the Motor Brake and use an ohmmeter on the lowest setting to measure the coil resistance (impedance) at the 2 pins inside of the motor brake socket. The spec is 25.5 ohms, and 28 ohms or so is typically seen. Under 20 ohms and it starts getting intermittent, totally failing to pass test at 19 or 18 ohms.
You can buy a new one for a fair price from a seller some of us have bought from before...
https://www.ebay.com/itm/301992451277
Bob