The problem could be the connections of the video mini coax's to the backplane under the bias module, or possibly tarnished pins on the CRT. Have you absolutely verified this problem is not an external connection problem by swapping the red with another color on your RGB connections. Sometimes people immediately assume the problem is internal before exhuasting all external possibilities. The input module can also cause these symptoms.
We have also seen bad capacitors on the video control board that can cause a channel to cut out. You can start by reseating every board in the card case, with ac power disconnected, then swap modules if you can borrow some; the vcm and input cards for sure, the red neck board maybe. If it is flaky on the crosshatch pattern then the problem is inside the ECP. You can also tap on connections and modules while the set is running, use something insulated like a heavy wooden dowel rod or plastic rod.
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