Thought I'd put this up here in case anyone else has the same problem. I had a Samsung LNT4671FX/XAA that would work from cold, but would quickly develop video artifact, mostly in the lower half of the screen. If left going, the lower half off the screen would eventually freeze and then the TV would begin endlessly rebooting until I unplugged it and allowed it to cool. Removing the back of the TV lengthened the time it took for the artifact to show up, so I was pretty sure it was a heat issue. I had a few spare boards from this model so I first tried changing out the power supply with no improvement. At this point I was figuring main board, but on a whim, I switched out the intermediate video board (BN41-00944A). That solved the problem. I was surprised as I didn't know how much two way communication there was between this and the main board. Apparently enough to cause the TV to reboot if the intermediate is acting strangely!
Here's a pic of the board:
Here's a pic of the board:
