If you're adjusting the Screen setting "up" each time and it repeats the same problem, do you eventually get as far as the adjustment travels? If so, what do you do to get it to be visible again?
Wonder if your Brightness (Black Level) pot (on remote board likely) is dirty? A lot of people on here recommend swiping the troublesome pot all the way left and then right several times in case dust or crud is causing poor contact.
Have you capped the chassis before?
Also, you can visually inspect the bottom of the monitor chassis and look for bad (cold) solder joints and maybe try reflowing any you think look suspicious and also the joints under the flyback transformer.