might be getting too hot, but generally thats a sign of your GPU not being able to effectively complete it's calculations, IE not enough voltage to make it completely stable...if you could run prime95 for your GPU it would have errored is basically what I'm getting at.
I've had that happen plus I've seen a lot of people thats had that happen...there aren't any artifacts and things should be going smooth, but yeah the GPU just can't effectively do it's calculations and it's generally lack of Voltage to the GPU or getting a bit too hot