this happened once before, guy named Jim in Cinci or some such. He never followed up on our suggestions to see the issue through ot the end.
At any rate, it's either your video input board, or the video neck-board on the back of the tube.
Couple of things you can try.
1) slide the primary video input board out and swap the Red and green cables. If the problem moves to the green tube, you know it's this board. I have a document that walks you through this ,very simple.
2) the other thing would be to swap the video neck-boards between Red and either blue or green. See if it follows this board, this is more complex but not rocket science. Shown here