I usually do 12 hour+ stints before I consider a system stable
but after an hour, it freezes.
You really thing I should need more than 1.35 vcore for 4.0Ghz
Oh. You're trying to run it for a longer duration but can't.
If your doing the small FFT's, and it's failing, you'll need more vcore.
I don't remember my vcore exactly for 4.0, but 1.35 sounds about right. Maybe your just a bump or two away from stable.