I can't tell for sure, but here's a thought...
Since the issue looks like analog but you discarded that possibility with your "control" machine, the only thing I can think of (you checked the board connector, didn't you?) would be the path between the analog board and the place of actual video generation.
Video output is digitally generated at pin 13 of UG6, then it goes thru R21, R22 with R20 and C40 between that signal and ground -- these passive components are worth checking, IMHO.
However, a properly timed but weirdly shaped sync signal might have similar effects... VSYNC goes straight from pin 14 of UG6 to the AB, while HSYNC has R24 between pin 13 of UG7 and the AB. Check these too.
Hope this helps,
Since the issue looks like analog but you discarded that possibility with your "control" machine, the only thing I can think of (you checked the board connector, didn't you?) would be the path between the analog board and the place of actual video generation.
Video output is digitally generated at pin 13 of UG6, then it goes thru R21, R22 with R20 and C40 between that signal and ground -- these passive components are worth checking, IMHO.
However, a properly timed but weirdly shaped sync signal might have similar effects... VSYNC goes straight from pin 14 of UG6 to the AB, while HSYNC has R24 between pin 13 of UG7 and the AB. Check these too.
Hope this helps,