setInterval - change INTERVAL dynamically with external file?

Hello, I'm using this code to change the speed of a single MC, where "slideshow" is the instance name of my MC.

I want to change the Interval dynamically with an external text file. Actually, the client wants to be able to alter the speed of the slideshow, but not the entire flash file.

playID = setInterval(playMovie, 100);
So I want to be able to plug in "100", "50" etc. in my text file.

Also, is there a way to stop the setInterval dynamically as well? They want to be able to stop, so there is NO interval at all (completely stopped)?

--------------------------------------------
function playMovie() {
      slideshow.gotoAndPlay(slideshow._currentframe+1);
}
playID = setInterval(playMovie, 100);


help is appreciated!!
Thanks!!!