Blocks appearing on my tv screen.

Question

I have a vizio galleria tv. I've had it for a few months (it's a referb) and now when I am watching it, small squares with no picture are appearing. They are no bigger than say your thumb but they will appear and the move to a differant part of the screen. then some times they aren't there at all. It's very random as to when they show up and where on the screen they show up. Any ideas on what this is or why?