Kees Theremin has a wandering pitch

I bought a Kees Theremin a few years ago and the pitch seems to steadily increase over time, no matter how long it has been warming up. Is this a common problem with theremins in general or would buying a higher end model like the Moog Etherwave resolve that issue?

It doesn't sound appropriate. You should contact Kees about it.

I had let him know about the problem a few months after I bought it (about a year ago or so), and he said to send it in, so I did. There does seem to be a slight improvement but it still wanders and I don't know why... I suppose I could always contact him and send it in again if he's made some major improvements in his setup.