Big briar linearity

Excuse me, I have a problem with my second-hand big briar. I found that the high notes（C6-C7）linearity changed when I needed to keep my zero beat distance. When the weather became humidity and I turned pitch nob clockwise, the high notes disappeared. This is a big problem for me to play theremin. Especially, I need to think about the relationship between distance for each note and the highest note. ls this a regular circuit problem for big briars? 
Thank you, sincerely.