This is a common problem. I suggest you get another monitor... LCDs are excellent on the eyes, compared to CRTs...
There is something called Eye-strain that you can get from CRTs and they HURT. Basically what happens is you start to see multi-colored spots and you begin to loose focus on the screen. Followed by that is a nerve wrecking headache that lasts for about 30 minutes. That's evidence that you gotta change your CRT! I had that same problem and I had to get rid of my monitor quickly! So, almost immediately, I bought a new monitor.