I can't speak for why that would be happening. I don't have any data on plasma units interfering with AM radio signals. If you think it's a power issue, I might suggest using a power line conditioner for the television.
I bought a Samsung PN58B650 several weeks ago and noticed it caused a loud buzz on EVERY radio in the house when on AM. Samsung sent a service rep out. They tried to initially say it was a problem with the cable but we disconnected every I/O to the TV and even plugged it into a completely different outlet with no reduction in the noise. They said they had never seen that problem but would be willing to take it into the shop and see what they could find. That didn't give me a lot of confidence and since it was 2 weeks old I took it back and exchanged it for another set of the same model. I plugged this one into another outlet with nothing connected to any of the I/O and the new one caused the same problem. Is there a know problem with this set or is there anything in my environment that could be causing this? I have seen some older forums relating to plasma TV's causing HAM/AM interference but nothing like this. It appears to be across the whole AM spectrum and does vary with the picture.