Hello!, my Theremini sounds shaky at low frequencies and even, the passage from some notes to another is longer than normal.
I have calibrated it repeatedly, changed banks and the problem continues.
What I can do?
Same problem I am faced with my dad’s one!
You should contact tech support, that sounds to me like a faulty IC on the pitch antenna board
I think it is not the pitch board, but insufficient grounding of the instrument.
Also read my post here: https://forum.moogmusic.com/viewtopic.php?f=5&t=28394
The Theremini power supply is special because Gnd (outer contact of DC plug) is connected to the PE (protective earth) contact of the mains plug.
This is not the case with many other power supplies.
I also noted that not all old style transformer power supplies (heavy wall warts) are suited because Theremini is susceptable to ripple on the DC (you hear a humm in the Audio).
Only good quality regulated supplies with Gnd (3-wire mains cord) will work.
Had a similar problem so returned it for a replacement then the new one started diing it. Turned out it was the power supply on my laptop which i was using for the editor. As soon as i switched it off prblem solved.
Can confirm, I ran into several of the aforementioned issues with my Theremini: both power supply ones (the supply was too weak, or had an inappropriate frequency, and this made the sound of the instrument “blurrier” and transitions slower) and insufficient stability issues due to, well, a wobbly table. Sometimes, issues come from something so simple that it becomes hard to fins 
it’s like the term “bug” for a computer issue that was coined by an American engineer that ran into issues with one of the first electronic computers in the 40s because a literal bug had climbed into the machine and had himself fried in it…
Had to google that one.
it was Grace Hopper, actually, who found the burnt moth on a relay inside the computer, in 1945!