So I finally managed to pin down the Uncore clocking problem to the setting which is causing me to not be able to clock past 2x + 1 in reference to DRAM. If the memory multiplier is left on Auto the Uncore cannot be clocked past 2x + 1 and POST fails with looping F3/68 codes even if Auto multiplier results in correctly rated DRAM speed within specifications. However, setting manual multiplier to say 2:8 and then clocking Uncore independently works fine. I managed to quickly clock Uncore to 2x + 4 in reference to 1600MHz DRAM (4GHz Uncore). I am certain this is the problem because I can now reproduce it at will just by flipping back to Auto DRAM multiplier instead of manually setting one.