Theremini calibration/stability and treble issue

[gulsahinfrance]

Hi all,
I have a theremini since last year. I did not play it for couple of months and now I am facing calibration/stability and treble issue. After the calibration, in some minutes I am loosing the pitches so it is not stable. On the other hand, on the lower notes, I am facing treble issue. I do not have any object around it with 1.5m minimum distance. Besides, I was always playing it on the same spot and before this never happened. Does anyone have an idea what could cause this? Many thanks in advance.
Gulsah