Sounds like your GPU is either overclocked and overheating or just overheating. If it's clocked, lower the overclock because this game will really push your GPU. Otherwise, try cleaning your GPU or improving the cooling in your case and/or GPU. Make sure you have the latest drivers installed as well.
Do you mean that you get the problem after 8 hours of playing continually or that you are getting the graphics anomalies all the time when you start the game now?