The GPX and the GPX.WCI are two of the most widely used grinders MPE makes. Because of their popularity, it is not uncommon for customers to seek support for many of the same issues over the lifetime of grinder ownership.
These videos provide expert help for common GPX support issues.
Grinder Shuts Off Randomly
Make sure the right voltage is getting to your grinder. If the voltage is too low, the motor draws more amps and can trip the electrical box.
The Rotor Isn't Coming Off the Shaft
There are two screws you can put inside the center rotor insert (the name of the part) to safely dislodge the rotor from the shaft. Stuck rotors appear to be more common with single phase grinders. Because the tolerance between the rotor insert and shaft is very tight, over time the shaft's end appears to bend slightly. Use a file to level out the shaft's edges until the rotor can fit and release easily from the shaft.
The Grinder Is On, but Coffee Isn't Getting Ground
Your 3 phase motor is wired in reverse. Switch any of the two terminals (not the ground though).