I am currently playing Far Cry 3 and recently I experienced some lag that was completely random but then just as it came, it went. The next thing that happenned was that the shadows in the scenery and etc, were blue instead of black but that wasn't the only case, sometimes when I enter caves the colors are just not right in general.
My gpu, cpu temperatures are completely normal (45°C cpu and 56° gpu).
System specs: Intel Core i5 2400, asus 6850, gygabyte GA-P67A-UD3-B3, 2*4Gb g.skill sniper (1600 mhz), sentey 550W solid power.
Your problem seems to be a defective graphic card. one thing you can do is to uninstall, the driver, then the graphic card, then again install it . Make sure that the connections are well matched , so the card is well sit. Reinstal the driver, and updated the drivers from their site. If the problem still exists, then if you can exchange your graphic card.