No one mentions it could be yuor fault for pulling aggro? I'm not saying it is, but could very well be.

Phase transitions can be tricky healing at first, any hots rolling will give you aggro as soon as the next phase drops. First thing I would do is make sure yuo have no regens/medicka 2s rolling when you go through a phase transition. I assume you all stack in the middle, so that he can grab aggro off of everything (at least intially).

Hold off on healing, if you can, until you see your enmity bars go green after a drop, once they are green, you should be relatively fine for aggro until the next set drops.

If your doing that already, well, its probably the tank.