A bad jog wheel on the Akai MPC2000XL will cause your settings to jump
around or not change at all.
The component (left) that translates the jog wheel's rotation into on/off signals
for the computer is called a rotary encoder.
The jog post (or stem) of the encoder can become loose over time and pull out
of the encoder's frame.
Fortunately, installing a replacement encoder isn't a huge deal if
you're handy with a screwdriver and a soldering iron.