I really don't think that upgrading the power cord will make a difference.
If by slight chance you do get a slight improvement by upgrading it, it could be due to:
1. The TV draws enough power that heavier conductors are helpful. First, try a heavier gauge extension cord if applicable. The power cord to th TV itself is quite short, about 7 feet, so the loss of power in thin wires is quite minimal.
2. If you can find and upgrade to a shielded power cable, less interference may be picked up by nearby video cables.
Video hints:
http://members.aol.com/ajaynejr/video.htm