You may want to start saving up for a new display, it sounds like the high voltage is acting up on yours.
It should be safe to use until it fails. Catastrophic failure are extremely rare and have been for some time. You also have enough common sense to know that if you did see smoke or fire to shut it off and not use it again.
How long it lasts depends on what the problem is. Meaning if it’s a loose connection it will last longer than if the component is beginning to fail. Again, I would not recommend taking it to a shop. Even if you find a top-notch place that could do an adequate job replacing the power supply board, phospher wear with your display is a factor here also.
Regarding new displays… CRT vs. LCD is always a personal choice. I’m a fan of LCD for many reasons, and from a practical point there is one very big advantage. My kitty doesn’t have room to perch on top of the thin bezel, and I’ve yet to own a kitty that doesn’t love the top of a CRT.
Hope that helps, and best of luck with it.