Well....it's unusual but....
Since the problem seems to occur about an hour after running fine, we know the 6 or 7 blink code doesn't seem to be an issue with the high power/high current/high temperature boards you just replaced.
I got a couple pictures here to help you understand what that 6/7 blink code is supposed to tell us --
You would think those 3 boards are the source of the problem. They fail a LOT.
But once in a while, they could be operating fine, but the monitoring signal could actually be the source of the problem and since it fails when warm, we may want to try heating up the other two boards when they are cold and see if we get the TV to shut down and display the blinking fail code when they are cool.
This picture shows the entire fail code path. There's a D board and a DG board, and I suspect the D board FIRST. It shuts down the TV set. I think the DG board just recieves the signal from the D board, and makes the lamp blink on the front. I don't know 100% if it is the Master control computer or if the D Board's computer is the Master in this situation.
But if you warm up the D board....and the TV shuts off, then I'd say the problem is on the D Board.
But I'd also want to let that cool down, turn the TV back on....and then heat the DG board with a hairdryer.
We know it's heat related....so a nice icy cold, room temperature TV set will react when heat is applied to the faulty board.
Some will react almost instantly, others take 5-10 minutes. But you can't heat up one board, then heat up the other. You heat ONE up.....does or doesn't fail. Turn the TV off. Let it cool for an hour. Turn it back on, then heat up the OTHER board. No failure?
That's when I try the power supply board, because that SUPPLIES power to ALL boards !
Geez, sure hope you have a female in your home. I hear a hair dryer running in my house from time to time, but I have no idea where it exists.
You may be in the same boat as I am about hair dryers.
But, I usually set it for medium or high and wash the entire board with hot air while it's running. Don't put it right over an IC about an inch away.....that can hurt parts. But 4 -6 inches, moving it around....that'll heat up a board pretty quick. Try the D board first for about 10-15 minutes, then shut the heat off....let it absorb the heat and watch it another 15 minute. Does it fail?
If so, shut it off, try that SAME procedure on the DG board.....same time cycle. See if it reacts.
Which ever one causes a reaction.....COOL down the TV for an hour, and then REPEAT the HEAT a second time to VERIFY it reacts the same way.
Note this.....heat failures usually occur at a VERY specific temperature. Your hair dryer may very well heat it up so fast that the temperature shoots right past the fail point. That's why you heat it up, then take the heat off and watch it.
That's also why you run the heat test only on one board at a time.
Here's the locations of the boards in question --