problems with NEC AccuSync 900, any Ideas?

[mitresquare]

I have an NEC AccuSync 900, CRT, that for some reason is randomly changing to a greenish yellow hue. Sometimes it switches back and forth to this hue for a second or two and then goes back to normal. Some times it seemingly stays that way until I move the monitor some or smack it a little. Anyone have any Ideas what is going on or how to fix this permanently? thanks in advance.