When I plug my electric guitar into a multi-effects pedal and from the pedal into my amp, it works perfectly. But, when I use the same guitar, pedal, and cords that were just functioning correctly, but connect to a PA instead of an amp, the volume fluctuates - almost like a tremolo, but not at any particular tempo.
To be clear, these strange fluctuations happen regardless of the effect I choose, but none of the other instruments/vocals through the PA are affected. I can't figure out what this could be, especially considering that the exact same setup works fine through an amp. Thoughts?