It sounds like a bad connection. It may be in the ribbon cable plug that connects to the Variax main board rather than the piezo element itself, so I would suggest re-seating the plug as well as making sure that the mechanical connection between the piezo pickup element and the bridge metallic parts is clean and free from oxidisation or other grime that might give rise to a poor electrical connection.
+1 to nick. try cleaning and re seating the ribbon cable. If that doesn't work you might need to take the guitar to an authorized service center to get checked out:
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