This Thing You Call Aggro.

[Mirajj]

When anything moved the least bit toward the healer, the hunter tried to solo. BAD IDEA. Our mage and I were trying to CC the sucker, especially previously sapped/poly'ed high priority targets.
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It sounds to me like one of the main problems you have is that no one in the party seems to be sure of their secondary role. This would be solved easily by telling the hunter at the start "Don't worry about the healer, our mages will take care of anything threatening the healer." Or telling the mages "Don't worry about the healer, the hunter will take care of anything threating them."

That way, you have a defined healer protector, instead of all the suddden 2-4 people pulling their DPS off the tank's mob to defend the healer.

If the person defending the healer needs an extra hand dealing with the healer's mobs, they can call for it, then either a secondary healer assist, or everyone piles onto what's attacking the healer.

Edit:For a bit more clarity of thought.