[butvacuum]

Ruled out the monitor(s)? There's been cases where they've backfed power, and they certainly backfeed EMI as well. And, it could also be tied to FPS- assuming gsync/free sync.

If you have a multimeter its probably worth double checking if the case is low resistance grounded to the end of the cord. I'm assuming you have checked already, but as a shock hazard it bares repeating.