A very common problem in this model.
This model uses two of the STK-392-570 convergence chips, a lot of these the chips get flaky, cut in and out when cold or hot.
What happens is both are mounted on a large metal heatsink with a white grease to transfer heat on the back of each chip (see photo of similar chip) and after some years the grease dries out and the chips start to go.
In every case with intermittent loss of convergence (looks three d) it is one of the chips. They are located on the middle board in the set and available as a pair (I can tell you where I buy mine) for under 20 dollars, you buy them local and expect to pay 50 dollars.
This can be a do it yourself repair if you can solder etc, I have start to finish, step by step instructions for this which also tell you everything you will need to do the repair.
Sooner or later one of the chips may short and set will then not come on at all.
Before you rate my solution here let me know if you have any questions and if you want the instructions we will find a way to send them to you.
Regards, SD TECH
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