Double, triple, quadruple, fivetuple, and hexuple check that the cord is screwed in tight on both ends. Every time I've seen discoloration on CRTs, it was a loose cable (or a visibly bent pin on one of the d-sub's).
I'm assuming you've already tried a degauss too, that might fix it. Any speakers near your monitor may also cause discolouration, or electric motors (i.e. a fan).
got any other electronics near it? like cmdr said, speakers can cause discoloration due to the magnetism in them along with anything else, Try a degauss.