I have an MSI GTX 680 PE coupled with an older Core i7 860 with 16GB 1600mhz RAM (on Corsair HX650W) and I was running into similar issues, low/unstable frame rate and even crashes/blue screens. Turns out that my CPU was causing it since it was overclocked from stock 2.8 to 3.36ghz on stock voltage, I reset the BIOS, raised the base clock to 160 and no more issues. Far Cry 3 is very CPU demanding, task manager shows up to 85% usage on CPU in my case, check to make sure you didn't touch the voltages or accidentally mess up any other settings. Hopefully this was helpful. Good luck.