The other thing that may be hurting you is your driving. If you aren't driving him very well then not only will you get a lot more damage for your healers but your dps suffers as well since you lose the armor slower. Make sure the person doing it is someone with good raid awareness so they always know where the inactive and active volcanoes are (you shouldn't have to run over an inactive volcano very much at all) and that when there isn't an active volcano up you are always turning him towards the middle or towards a group of inactive volcanoes so that when he activates one you aren't out of position. And of course make sure that your raid is listening to your driver and not over-reacting to his calls. If you are still having trouble with that there are addons out there that can tell you who is targeting what leg so that you can make sure people are doing the right things.