NaN is a floating point error, where the result is Not a Number (Hence the abbreviation). Typically this happens when the results of a calculation rely on a previous calculation, and there's been an error somewhere. For example, trying to divide a number by 0 will lead to a NaN, as infinity can't be represented.
That suggests that the card isn't functioning properly and you're getting errors in it's calculations. Heat, power supply problems (either main
PSU or oncard power), card
RAM issues and a dying card could all cause this kind of error!