I have to agree with the bad monitor suggestion. If it's happening intermittently I could be a broken wire in the cord. I had an old monitor where if you jiggled the cable the screen would change colours. I had it on one of my servers that only ran a text terminal so it was fun to play around with different colours. It eventually died... I wonder why?
Do you mean NEC monitors?
I had a horrible experience with NEC. Sit back and let me tell you a tail of woe and misery.
I had dual head running on a pair of 19" NEC CRT's. After about 8 months both started emitting a high pitch squeal noise whenever there was a lot of white on the screen. I called NEC up cause they were still on warranty.
They sent me refurbished ones (part of their warranty policy or some such nonsense cause they didn't make that model anymore) and I sent the old ones back. On one of the "new" ones they sent me the screen would twitch intermittently and the other one had an awful yellow tint to it.
I had them send me two more sent the other ones back. With this pair one had some cruft stuck behind the glass or the aperture grille was damaged cause there were several black spots, like dead pixels on an LCD but this was a CRT. The other one was even more yellow then the last one.
Sent them back again and got yet two more. By golly these two actually seemed to be okay at first. But after a couple weeks one of them started twitching, at which point I did too.
I had been patient and polite up till this point in dealing with them but I had reached my limit. I called them up, demanded I speak to somebody in charge gave him each RMA number they had given me and the serial number from each monitor so he could see the history of my dealing with them and then berated him for about half an hour on how unacceptable this level of service was and how disappointed I was cause I use to think that NEC was good brand.
I then went out and bought a pair of 19" Samsung LCD's. I now warn everyone who comes to me about computer advice about getting NEC's. Consider yourself warned.