Hey, Antoneo,
I've got a CTX 1785 GM that has done different odd things like getting dark or going completely black and also losing focus. I've had it since Sept 1995. I can't say this is true for all monitors, but my CTX (and a DEC trinitron I used to have at work) have a plethora of additional adjuctment controls INSIDE the case. These are the fine tuning controls that the factory and technicians use to set the basic monitor image. They are extremely sensitive, but if you have a steady hand and can get into your monitor case, you might be able to breath a little more life into that monitor. Just be real careful not to stick your screw driver anywhere that might elicit a shock.
Getting in the case saved me from going out to buy a new monitor twice now when the image went bonkers and I could not clear it up with the onscreen controls. Good luck and be careful if you go this route.
CB