A year or two ago, there was a member who had interference to his KU reception on a Ku dish, but only when his C-band dish was on a particular bird.
He had a heck of a time recognizing the combination of situations that caused the problem, as you can imagine.
What he had, was feed-through in his diseqc(?) switch, as I recall.
To test that possibility, you could pull the C-band signal from your switch, but do be careful to do it while you are powered down, so as not to blow out the diseqc.
And, a drawing or clear description of your wiring may be helpful, too.