|It can commonly be one of, or both of, two things that could cause your symptoms....|
- the backlight is nearing the end of it's life and the voltage inverter is okay but is detecting there is something wrong with the backlight and is switching itself into a lesser voltage mode after a few minutes to protect itself - in that case before that happens you often have already been seeing a tint to the display sometimes for a while - when the screen goes black if you turn the brightness all the way up, you can often make out very dim images some of the time, but not all the of the time, depending on what is being displayed - e.g. you may be able to make out the Windows logo when Windows first loads but not the desktop.
- the voltage inverter is defective - in that case, when it goes black you see no image at any time regardless of the brightness setting. However, some voltage inverters may switch themselves off rather than going into a lesser voltage mode when the backlight is dying.
If you still have a CRT monitor, I advise you to use it until it's image is no longer good enough.
Many people believe an LCD display will be usable longer than a CRT display, but that's not necessarily true. Some brands of LCD displays are warranteed for only one year regarding the backlight, though for better brands that's often three years. I've had CRT displays, which commonly had a 3 year warranty, that lasted as long as ten years. We frequently hear about LCD displays that have developed symptoms like yours in much less time than that.
Even recent LCD displays are not as good as the later CRT displays.
LCD displays cannot display rapid motion as well as a CRT display, and they always have a "native" or "optimal" resolution they look best set to - when set to other resolutions they don't look as good and text is often not as clear in those other resolutions. CRT displays are equally clear in any resolution they can be set to.