Hi folks,
bit of a technical question here.
I got an old Big Briar Etherwave modified with an ESPE01 module. I have it since several years, it was 2nd or 3rd hand, and since it's not my main Theremin I always used it quite randomly and not on a regular basis, so I can't recall if the problem was already there and gradually became increasingly noticeable, or it's something that started at a certain point.
Now, I'm starting to use it more, and I noticed that the sound feels quite spurious. Not like it's clipped, but it feels like there's a lot of weird artifacts in the noise floor, which makes the overall sound quite fluffy and chipped. This is more noticeable while moving the waveform knob, as the harmonics seems to behaves weirdly and in a slightly unstable way.
Here, some frequency plots where it is possible to notice the overall spikeness - indeed, it sounds a bit FM-ish (brigthness CW and CCW respectively):
I have inspected the circuit board, and other than some old moisture stains which I cleaned, there do not appear to be any visibly damaged components. The wires connecting the ESPE01 to C2 and C6 also seem fine. I also tried to retune it (leaving L5 untouched, even if I don't know if the previous owners mangled with it), but still no success.
I read somewhere that this behaviour could be related to some coupling capacitors not working properly. Could that be the problem? What can I try to solve it?
I would like to be sure to doing the correct operations before swapping all the caps! 
Thank you so much!