maybe AMD 790FX chipset is conflict with your GTX 260.. i recommend you that change to ATI graphic card and see it has any problem or not. In addition, if you wanna using Nvidia Graphic card, I recommend you that use intel chipset, or Nvidia nForce (680i, 780i, or 790i) chipset for best performance...sometimes people use AMD chipset with Nvidia BIOS, it might cause some problems that we cannot explain.