The problem is almost certainly heat, judging from the time and usage you relate. The consistency of failure probably comes from the same portion of memory getting the hottest each time.
To fix it, you need to get a.) more cool air into your system, and b.) more hot air out of your system. I list a.) first because it is common for me to see up to 7 fans blowing out of a gaming box, and zero or one blowing in. There needs to be some balance.
For a hot system, you need at least two fans blowing in, usually one in front and one in the side. You may have to flip the side fan around as many case mfgs bizarrely ship with it installed to blow out. Input fans can run quietly at faster speed because most of the sound goes into the box.
You can run output fans at lower speed, and have more of them, and/or larger ones, in order to get the airflow you need and still stay quiet.
Another alternative is to upgrade the cooler on your video card, and/or add heat sinks to the memory of your video card (assuming the memory chips are exposed).
Finally, pay attention to airflow within your box. Having a large, flat IDE cable lay across the side of the video card can seriously restrict airflow, or may even reflect hot air vented from the video card cooler right back into its air intake. Liberal use of cable ties, round cable, and careful routing will improve airflow. You goal is to have nothing in between an input fan and the video card.