2 things here:
First, you give your health movie clip a name, let's say health_bar. Whenever you want to move the playhead of that movie clip you must targetting that name. (health_bar.gotoAndStop(frame_number)) or else you move your main movie playhead instead of the health_bar.
Second, you don't have to name each one of your keyframes for the gotoAndStop to work. You just enter the frame number. And that way you don't have to have 100 keyframes!
I hope I made it clear.