ouch! 1.6v is a hefty volt. i just got 4.0Ghz with 1.52V tops, CPUNB at 1.31V i think. Are you running an AM2+ board? just with you using DDR2...it may be limiting you on the power front? I think AM2 boards have a slightly different VRM setup which can limit things.
I'm on an AM3 board, but when i was overclocking mine, i hit a wall at about 3.75Ghz....then it needed much more Vcore, (from about 1.4V up to 1.52V) and I had to lower my RAM speed - though to be fair i couldn't use multiplier overclocking - I was using the FSB (i've a locked chip, my FSB is at 267Mhz). Maybe try loosening your RAM timing, try a lower frequency for the ram, and get the cores rock solid on overclock, then work on your RAM speed. thats how i eventually got to 4Ghz I had to leave my RAM at 1424Mhz, it wasn't liking 1667Mhz +
PS - can you try setting your RAM to 6-7-7-21? that'll loosen the timings a bit, and might help.
PPS - had a quick look at the Exception Code: c0000005, its trying to access a nonexistent memory address, either caused by a corrupt heap, or misaddressed memory.
= either your CPU cache or the memory controller are corrupting data (IE run out of overclocking steam). Up your CPUNB a little, and check out overvolting your DDR2 RAM, see if it helps. (Sorry, i don't know the safe limits for DDR2 voltage. Can anyone with DDR2 experience pitch in?) Either way, it a hardware exception that the processor is throwing, due to misaddressing memory.Edited by latelesley - 9/15/13 at 9:25pm