On the principle that it could be RF (EM) interference inside the box have you tried shifting cables and/or shielding them? I know that the EM noise at the back of my TV can play hob with the audio signal, and that having a VCR in the same circuit messed with my TV signal qualty. A little bit of shielding and grounding can make a huge difference...
Silly (or maybe scary) but true story... At my previous job one of the guys on my floor had a 3 foot square area of aluminum foil behind his monitor with a little wire attached and running down to a grounding point. When asked what it was for he'd unhook the wire and looking at his monitor could literally make you seasick and nauseous. Seems there was a piece of equipment on the other side of the wall that produced a signal at just the right low frequency to cause a very nasty slow flutter. Urp!
