While your GPU may not be overheating, it looks like the video RAM is, overheating video RAM is the usual cause for artifacting. Download GPU-Z and run that in the background, it monitors temps for the various parts of the video card, including the video RAM. If the video RAM is overheating, you may have to remove the card's cooler, reapply the thermal paste to all the appropriate areas, then reattach the cooler. Some of the older nvidia cards out there are having problems with the thermal paste losing its effectiveness over time, leading to overheating issues, even if the cooler is perfectly clean.
If there are no heat issues, it does seem that your card may be on its way out.