It could be a grounding issue. If the PA and the lights are on the same circuit, you could be in a ground loop, which will certainly cause a buzzing. Since it comes and goes though, it sounds as if the loop may be a short/intermittent connection somewhere.
Normally, I'd recommend lifting the ground in most cases, but on intelligent lights, I really don't recommend it. You could possibly lift the ground on the PA if it is on the same circuit as the lights (it's up to you though, because there is always some risk in lifting the ground, but it usually solves ground-loop issues).
Just some ideas, not sure if it will help. Just be careful and think things through.
Best,
-Tech