It sounds heat related. Did you keep up with the maintenance? Such as a little canned air on the vents once a year?
We bought this tv probably 5 years ago. Just in the last month or so, we would leave it on for a while while doing something else and Id notice the screen was black, but audio still on. Id need to restart the tv to get normal picture back. Then this week it will only last about 10 minutes before the screen turns black. I can tell when its about to happen because the screen will almost rotate a small amount, like its dropping, and I can see some white lines at the top, with a loss of picture quality, then turn black. It seems to be getting more frequent.