Hi Graham

In one sense you confirm what I say - keyboard settings fine until the voltage goes below the threshold, then vanish....

But I guess we'd have to go over and measure the OP's gear. the $64,000 question is whether it does it at home. If not, then it HAS to be something peculiar to the location (ie voltage or local RF).

LOCAL RF - I think I just got it!!!! Is there a compressor in the signal path? If so all you need is RF interference outside the audio band and bingo, the signal ducks.



Back to your festival....

Your mixer had a stabilised power supply, yes? So it wasn't that.
Your FX (and controllers) ditto.

That leaves power amps.

What power amps were you running?