Make sure your power settings (usually found in screen saver routine) do not turn off the monitor after idle time. I am also thinking some of your problem could be heat build up as this sounds more like a hardware issue with the video card. Try reseating it and make sure you have good air flow inside the case. You can try running the PC with the side panel off to see if the same things happens. If not, it could be heat related even to local components on the card.