if you are constantly having to recalibrate the joystick then it has a bad pot in it.
you can replace it, or just get a reman joystick. That should solve that problem.
the code 44 problem can be a bear to track down, i would attempt to do more circuit checks before i went throwing parts at it.
as for the code 84? that is the sensor on the drive unit (the pot on the top with the little cog belt). If the brake is not holding good enough when you start up it will throw that code. During the startup test the system will jog the drive motor to test the brake, if it moves that triggers that code. Check the belt, make sure it is tight, check the gears on the sensor, the set screws might be loose. And make sure the brake is set and not loose when applied. You can adjust it and tighten it up a bit so it will hold better. Just make sure after you adjust it that it is not dragging, when it releases it should freewheel with no drag.