I have an LP II (circa 2010) that sounds fine through headphones, but any time I send audio to an amplifier the output volume dips and surges unpredictably. I do not see “dancing LEDs” and I don’t notice any other symptoms. The problem persists when connected to any external speaker whether I use the “audio out” or the headphones out. I’ve tried various cables too. (Thinking this might be a ground loop, I connected the LP to a battery-powered amp, but that didn’t help any more than changing the rooms.) The only way to hear a clear, constant tone is to listen through headphones. It’s been years since I’ve used this on a gig and I didn’t notice it in the past. I have firmware version 3.21. Thanks for any replies.
The only thing I can think of to suggest is to exercise the knob. Not sure if these are self-lubricating though. I have an LP Tribute and noticed something similar with the filter frequency knob. I can’t remember if I got it to work better or if I just got used to it - obviously not an acceptable solution in your case!
Not sure the potentiometer is th culprit because it controls the volume in both main output and headphone output.
Thank you for your responses. This seems to have resolved itself after a few days and I’m not sure why. The potentiometer controlling volume seems to be the most likely culprit, though I don’t know enough about the LP design to know if it’s even possible: one knob controls both the headphones and the audio out levels, and only audio out was affected. The moog lives to play another day!
Good news! Hope this will last forever 
I just bought a used LP a couple of days ago, and I’ve had the same issue. Hopefully mine gets fixed on it’s own too
Did the sound start working for you as well. I just got a used and it has the same issue.