Making a graphic flash programmaticly

Alright, I am trying to make a graphic flash at regular intervals that I can control via AS3. I was trying to do this with tweening but I am getting bogged down because for some reason when I try to get the darn thing to loop it just won’t do it. I want to be able to change the rate at which the graphic flashes on and off the screen within the constraints of the frames per second so that I can simulate a light and sound machine. Ideally I’d like to be able program the starting frequency of the flashing light (say 15hz) and the ending frequency (5hz for example) and the program would gradually shift the frequency from one to the other over the course of the graphic. For right now I would love it if I could just programmaticly set the frequency of the flashing graphic within the limitations of the frames per second. Could someone please help me…