Set the cache to 3.5ghz and leave it alone for now. Dial in your core overclock, and be sure to avoid going over 1.35 vcore for now.
Once you find your stable max, add a little vcore, then start pegging the cache.
My 5930k initially was very picky about cache speed, but basically making the cache voltage just a little less than vcore, I was able to pull off a current overclock of 4.25ghz core, 4ghz cache, without going over 1.3 volts.
System Agent voltage will most likely need to be over 1.1 volts, as will IO voltages for cpu and chipset. But I also have 4 sticks of 8gb ram at 3ghz, so SA voltage needed a bump just to run those sticks at rated 3ghz, let alone running the cache fast.