Im thinking your health is a tiny bit low unbuffed to take alot of stacks. Something a guildie pointed out to me before, after the 4th snowbold is out, you have 1 minute to kill the boss before there is a 100% chance of a impale application, tick, melee swing (for a total of about 75-100k damage) in about .4 seconds. You might be getting owned by that combo. My guild prefers a 3 tank rotation with swapping at 3 on certain tanks (for lower gear) or 5 for geared tanks.
The only sovereign you can allow to rule you is reason. Talga Vassternich.