Thanks, I understand, but that's not the problem. It's hard to explain...
When I move the AT pot, the kHz switch doesn't move from its position (20 kHz). It simply goes into bypass while at 20 kHz. It also does this at 15 kHz, but less frequently.
It seems that when I move the AT pot, something shorts out or loses contact, on the switch causing it to go into bypass.
I could send you a short video