Doing a 25 naxx run today and on Instructor Razuvious I'm mind controlling as usual with some pug priest that has never done it. Now I explain the fight and everything to him and he says he understands. Starting the encounter mind soothe run in and MC. Do it and I start tanking him and calling out for when he should taunt etc. Though something weird is happening during MC. It randomly breaks and I have no adds on me or anything to cause the timer to go down etc. I was thinking it could be something with the other priest or quite possibly a bug?
Your problem is probably the range of the spell, MC has a 20 yard range and that is 20 yards away from you as you stand and cast. The spell is setup to break more often the further you move the target away.
Just try to get in close and try to move as little as possible.