quite possible your card's settings are too aggressively overclocked, under-volted, or something is causing it to overheat slightly (or any combination). A very common culprit is insufficient cooling.

Some cards come with a slight overclock out of the box to give them an edge, so even if you haven't tweaked it yourself it may be worth comparing your current settings against the reference specs at the manufacturer's site (AMD or nVidia) and adjust to reference specs if needed to see if it clears up. At the very least, you may want to adjust your fan speed to try to cool it better.