Sears 9" Color TV Weird Problem

[compucat]

After the set has been on for a couple of hours, the red in the picture gets weak. Flesh tones get very pale. The greyscale changes a bit too. When first turned on, the picture is fantastic and the color is rich and vivid. I don't think it is the picture tube because weak tubes usually improve when warmed up but this gets worse. Also the red does not appear silvery as it would with low emission. It looks as though it gets turned down very slowly. This is a 1972 solid state set that appears to have never been serviced in its long life.

Any thoughts? Sorry I don't have the model or chasiss number handy but TV Beta Guy has the same set.