Don't worry, they can only sheep you 4 times without you becomming immune.
Silence is four seconds, by the way and you do it in mid heal, not before.
PoM > Pyroblast > Instant > Instant > Priest Worries and starts the heal > Counter Spell: Silence > Cold Snap > Frost Bolt!
That doesn't work if they shield, which for some reason undead priests don't seem to do to me. They think that Shadow Word Pain is more important or something. *shrug*