Like everyone had been saying, practice threat management. Use these situations to get better at regaining and keeping hate throughout the fights. I always mark my targets and ask to have a mob put to sleep. This leaves me with keeping aggro on only two mobs which is easy. Just be sure to give the other mob some love as you drop the first one. If your dps is all over however, mark the target the highest dps is on and keep some hate going to both targets. Don't punish you healer when the dps won't listen. Besides, being able to overcome difficult situations makes you a better tank in the long run.