Sencore VA-48 issue

[DavGoodlin]

I was debating opening it up to fix this or just asking if anyone else had this issue?.
This unit works well for the color bars, but when you switch to crosshatch or dots, you see the pattern for a second, then it goes blank. I suspect something on the 500 board is going thermal and may try freezing some ICs

After using the color bars for 30 minutes or so, they disappear also. I have the schematic book and parts list, so I can easily locate and replace whats bad. I will start by checking the power supply but would appreciate any helpful hints- thanks!