1979 Magnavox Question

[jstout66]

This is going to be general question, as this set is not at my house, so I'm unsure of chassis/model number.
I do know it's a 1979 Magnavox (color console)
I've also never seen this symptom in a set.
After the set is running awhile, the brightness changes. It will get SUPER BRIGHT, and then go back to normal. Of course it's intermittant, but does seem to do it worse after the set has been on awhile. Also.. if this makes a differance, it doesn't matter if the auto color is on or not. (that set has an electronic eye that is activated when "videomatic" is pressed)
Otherwise.. the set has a FANTASTIC picture. It rivals my Chromacolor, which is unusual for a Magnavox set.
What should I be looking at?