Can you try another HDMI port, or have you tried this already?
And did you have any of the power problems I mentioned in my first post?
Is the TV set plugged into a power strip?
If so then try it directly into the wall.
Did you add anything new to the system, like an audio receiver or DVR/DVD/Blue-ray player?
And lastly, how old is the set, and have you ever changed the lamp since you bought the set?
If possible can you take a screen shot of the video problem and post it here. (you may need 3 more posts to be able to do this)