I had these fragments on monitor, not on TV, when I overclocked too much my cards.
I was going to say that I have seen these artifacts as well when too much overclocking has been applied.
OP, if you cards are prone to crashing I would back down the overclock just a bit, especially memory. If not and you can run for days on end with no problems other than the artifact, than I wouldn't worry about it too much and just soldier on.