I tend to doubt this is caused by something that you could fix in the service menu, but if you want to try, first power off your TV using remote, then Press Display + 5 + Vol Up + Power button simultaneously on remote. To save the changes, just press Mute + 0.
WRITE DOWN ALL MENU SETTINGS AS THEY ORIGINALLY ARE ON YOUR SCREEN BEFORE MAKING ANY CHANGES. Or take a photo of every menu setting with a camera if this is easier. YOUR SCREEN'S SERVICE MENU SETTINGS ARE UNIQUE TO YOUR SCREEN. If you screw things up by taking one too many settings too far, you will not be able to get back to your starting point without your own starting point as reference. Nobody here or anywhere else will be able to tell you what your factory original settings are, so do this before making ANY change. It's important.
Anyway, I think it is likely that there may be a failing part on your television's board. If I were in your shoes I would open up the back of the TV, pull the board up to service mode if you need to get a better look around, and try to go over everything with a flashlight. If you see any bulging capacitors that is a sure sign that they need replacing. Although capacitors do not need to be bulged in order to be bad, but you still might spot a problem. Other issues like bad chips, power regulators, etc., if you see anything that looks amiss, browned/burned parts. It might be worth looking for a local TV repair shop. They still exist, and if they have been in business for more than five years then you can bet that they have worked on CRTs there before. It doesn't hurt to ask.