This really doesn't sound very complicated.
You had the WS Board tested (and purchased a 2nd board), so the WS Board has been eliminated as a possibility.
You don't have the separate video card, so that can't be the issue.
Unless you have had mice chewing on the cables in your machine, the issue is in the operator's station.
You said you had the monitor in the operator's station replaced (although it likely only needed new backlights), but you did not replace the operator's station or any other components in it.
There are multiple components in the Operator's station that in theory could be drawing voltage down enough to cause an issue, but only one that is directly involved with powering the display and converting the video signal....seems like the best place to start.