/stopcasting then heal help.

[Bettysue]

anything after the g in stopcasting is ignored by wow, there is no way to specifiy which spell is stopped.

The unreliability you're seeing is most likely the fact that you are spamming the hell out of the button and stopping the heal as well.
Stopcasting is a curse at times you just have to train yourself to only whack the button once to save someone.

Another way that may work for you, and worked for me in the past was to make a macro solely for the purpose of stopping a cast then bind it to something and spam the separate heal button. This will most likely help you in those panic moments.