In the last frame of your animation, insert a blank keyframe. Then open the actions page (F9) and type gotoAndPlay(1); and it will loop continuously. (It doesn't loop when you play it in flash, you have to go to "Control" --> "Test Movie" (or CTRL+Enter) to see it loop.
edit: I totally misunderstood what you were asking. I know you can at least make the text small, then large, then small, then large frame by frame and at the very end use the gotoAndPlay code to jump back to the first frame in which the text starts flashing. Not really a pro yet so I am unsure how to do it a different way.
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