How to increase joystick ADC resolution?

[gtoal]

I've noticed that the joystick values increment by 4 points at a time, as you move it slowly from one edge to the opposite edge.  I did find the documentation about setting C81A to 0 to get the highest resolution but I tried that and it made no change.  Maybe I didn't clear the variable in the right place but I can't think where else I should add that statement.  A program that exhibits the behaviour is at gtoal.com/vectrex/joystick/AnaJoyTest.c.html - any ideas anyone what I need to do to get the full 256 point resolution?

thanks,

Graham