I have a Couple of EVGA GTX 680 SC's in SLI with XSPC Razor Gen 2 blocks on them in a custom loop, the temps max out @ 60c after 12 hours of Furmark yet they stubbornly refuse to stay stable even if I manually apply even just a 100MHz overclock to both the core and memory. I know they are capable of such a modest overclock since I can jack up the power limit to 132% and they reach such clocks when running 3DMark (2013) or running games such as Far Cry 3 or Crysis 3. I double checked the placement of the thermal pads and paste when I mounted the blocks, I even drained the loop and checked placement of the pads, paste and block again after leak testing the loop, the placement of the block, pads and paste are all where they should be. My PSU is a 3 year old Corsair HX850 Watt (OG) btw. Any suggestions as to why these 2 cards refuse to overclock ?