Why don't u start from small to big, instead of from big to small?
Try HTT = 210MHz -> 220MHz -> 230MHz -> 240MHz and increase vcore only when u need it. Prime or OCCT at least for 8 hours before moving on. That way, its easier to find the limits of your CPU, rather than the other way that you were doing.
Plus since you're getting a virgin 3500+, try putting it thru its paces first before gunning for higher OCs at lower voltages. Sometimes, burning in works too. My Opty 146 was only stable in 1.475v at 2.8GHz, but after 2 weeks of using it at such settings, I tried 1.45v and it passed prime for 10 hours. Soon, gonna try 2.9GHz and see if the voltage required for that (used to be 1.55v) is decreased too.