1981 Zenith System 3 Issue

[jstout66]

I have a 13" System 3, that I'm sure uses the 9-181 module ( I'll update if it uses a different one)
Here is my issue that just started. After the set is on about 1/2 hour, it shuts off. If you turn the switch off and back on it may run for 10 minutes to an hour before turning off again.
Anyone have any ideas? Zeno?
I'm wondering if it had some shut down circuit if the high voltage crept up?