I have the exact same issue where 3.8GHz is perfectly stable at a fair voltage of 1.38 (still really high but yeah), and 3.85 will cause my monitor to turn off and PC to freeze even up to a brutal 1.5V.
I've tried every which way to overclock to get 3.9. I am going to mess with the LLC more as that is the only piece I didn't know enough to play with before this post.
I agree with someone else that this is the best explained post ever. I will report later today with results!