We've had the same problem for the last 5 months with our Samsung 50inch plasma.
It had been sent in for repairs which cost $500. The power suppl was replaced but we're still sitting with the same issue.
Unfortunately I don't have any advice. We're stumped too...
I have a 40 inch Samsung LCD (LNS4095DX) that is less than 3 years old. The sound is fine, but the picture just disappears after the television has been on for a while. The problem occurs on any input, so it has nothing to do with the HDMI cable or anything else. This problem is being reported all over multiple forums on multiple models, but Samsung support (at 800 Samsung) is telling me that they have no knowledge of it and they won't fix it because I'm out of warranty. Has anyone found a way to get Samsung to fix this? It's absolutely absurd that a 2000 dollar TV lasts me less than 3 years, and considering that this is obviously a chronic problem, I find it hard to believe that Samsung wouldn't fix this just to ensure that they don't have significant issues with the reliability of their brand being called into question.
If anyone can help me, I'd greatly appreciate it.