Make sure you are only using the power adapter that came with your Theremini only. For best operation you will need a grounded outlet for operating your Theremini. If you are trying to play your Theremini but the pitch is very unstable and jittery, you might be plugged into an ungrounded outlet. 

For best results, you must find a grounded outlet. There is a ground screw on the rear panel of the Theremini that you can use to ground your Theremini.

Also, if other music equipment in your home / office / studio is earth grounded, you can plug the line output(s) of your Theremini into the other equipment to get grounded. Powerline network adapters may interfere with the Theremini.

If you are using them at your location and you are experiencing pitch instability, you may need to unplug them from the wall.