Can anyone explain this strange CRT fault?

[radiotvnut]

On an '88 Sears/Sanyo 20" TV, that uses a Zenith CRT, as well as other Zenith CRT's from the late '80's-early '90's; I've noticed a problem as these tubes age.

The emission will test OK, there will no leakage or shorts when checked on a CRT tester and the picture will be bright & sharp; but, the color temperature is constantly changing with changes in scene brightness, etc.

I've also seen some Zenith tubes display a blurry horizontal bar across bright areas of the picture. It's not as bad if the contrast is turned way down; but, it's still there. A normal picture will be on the screen; then, a blurry horizontal band will appear.

I know these two conditions are tube related because I've changed the CRT's in sets that had these problems and the trouble went away.

Does anyone know what CRT defect causes these problems?