Wow, that's an odd one! It seems like it's a PCB issue since it comes and goes? I would think that if it's a monitor issue it would always be there regardless of what's going on in the game. No? Bad Ram?
I'm just realizing, looking back at photos and videos is that the line wasn't there before I did the cap and B+ adjustment. After I did the cap kit the monitor needed some tweaking, mostly the brightness and the red. After that the screen picture was small so searching around I found Bobs page that showed the B+ voltage check. Checked it and it was super low at only 103v so bumped it up to 120v and the picture filled the screen better but not fully.
Maybe I just need to expand the image to the edges??