Use your Multi target rotation for single target. DnD is 1.9x threat modifier, you want it regardless of single or AE. I assume you're using Blood Tap to Howling Blast immediately and not waiting for your runes to come off cooldown.
I may be wrong, but it sounds like you're actually hitting Runestrike as a button? This RS should be macro'd into all your strike abilities:
I also use RS when ever available as with DC as an energy dump
I have this macro on all my strikes...so RS is basically automatic.
/cast Blood Strike
/cast Rune Strike
DC = Deathcoil? You shouldn't be using Deathcoil as Frost, you should be using Frost Strike. And you shouldn't be Frost Striking as soon as the RP is avialable...your RP should be high in case you get a string of RS procs. I only FS to avoid just burning RP...essentally only when i'm at 100% RP. Using FS as soon as you get 40 (or even 60) RP severely limits how many RS you get, as its unpredictable.
The exception are KM procs...a guaranteed crit FS is comparable to a RS so i'll burn RP for those.