What are your temps like? That case is very closed off and it's a mid tower cooler. You said no signs of thermal throttling, but stuttering is a sign. What are your idle temps in hwinfo64 and what do they go to when you launch a game, does hwinfo tell you it's not throttling?
Did you use enough thermal paste on application and make sure to remove the plastic film on bottom of cooler?
My first guess is it's getting too hot, and even if the cpu isn't throttling your vrms might be. Check with hwinfo but you can also open the side of the case and point a fan at it, see if that helps.