Q,
I've seen the undoctored video of McCain "screaming" with the crowd, and he didn't come off as the least bit unstable, out of control, or irrational.   

Of course, if you dislike him for other reasons ...

I just cannot imagine someone who otherwise admired Howard Dean and shared his political views abandoning their support for him because of a single incident like that - even if he HAD been the only one yelling.   Just not that big a deal.