I'm bad at holding packs. And I don't understand why.

Hello everyone. I need help.
Location: Utgarde keep. I have no problems with keeping the bosses. There are problems with the packs.

LVL 70
AP: 1566
HP: 10364
Hit: 2,03%
Exp: 3,5%
Crit: 14%
Def: 541

Spec: https://www.wowhead.com/wotlk/talent...1103-305-00502

Opening: Puddle (Death and decay) + 2 or 3 Blood boils. At this moment, a warrior enters a random target, breaks aggro and dies. It's looks like a kind of my guilty. Every time. Then I try to throw Pestilance. Blood boil or puddle. RDD, in the middle of the battle, can gets his aggro while damaging a single target. My AOE doesn't cover his single target damage.
I thought, that maybe, there is not enough puddle power. I gone to unholy until Impurity + 20% SPD. And maybe I've no enough of a spell hit, and get 3/3 Virulence. Nothing has changed. Some mobs of the packs keep dropping. I've run out of ideas on how to fix this. Help me please.

You are not supposed to keep warriors single target threat with your aoe threat.
Drop DND, spread diseases, then tab target mobs with lowest threat and use rune strike, death strike and, instead of blood boil, use blood strike on targets that need it. Get nameplate addon that shows threat nicely and makes targeting easy.
Use blood boil only to "collect" mobs, situations like adds on healer, ninjapulls, initial threat when your dnd is on cd.