I would recommend adding the nochanneling mod. I use it on my SPriest macro so I can spam my DPS macro for all my toons and not interrupt Mind Flay.

/castsequence [nochanneling] reset=8 Arcane Blast, Arcane Blast, Arcane Blast, Arcane Barrage, Arcane Missiles

Not sure if that is why you put in the /stopmacro [channeling] line for?