Hello Dave
When you turned on the TV, there was no visible problem for a few seconds, and then it appeared. This make me think it could indeed be heat-related. It really look like a frame buffer issue (memory or video processor). You had a good idea with the freeze-it, but when you tried, you had a static image displayed. If the processor doesn't update the video memory while you freeze the board, then the "corruption" may just be left intact in the memory. If the data doesn't get to the memory right, it would explain the corruption. Also, noticing one line out of 2 being bad on the display, and also noticing that there is 2 memory chips, one of the 2 chips could be "bad".
I would suggest doing the freeze-it again while displaying a dynamic image, like the screensaver or a live feed.
It could just be a simple thing as the memory chips not being heat-coupled to the back panel cover (missing or not-touching thermal pad for example, after all, proof being the stripped screw, it seem that the cover had been opened before).
Just an hypothesis...
Regards,
Eric