Don't want to sound like a broken record, but a little reading around here helped me. (will help you too!)
It seems like a little too much PLL voltage can actually cause more instability past 1.8-ish volts. At the advice of someone here along with the countless posts regarding their findings, I took mine off auto and manually entered in 1.55v.
Not only was I stable, it allowed me to drop my previous 4.5ghz @ 1.375 OC (which would immediately BSOD below 1.375) down to 1.335.
So, if you can see what your happy pll voltage is (for me, it's 1.55v, others 1.6 etc) it might allow you to actually get to the number you're looking for and require a tad less voltage than you're currently using.