Blink-effect with setInterval

I wanted to do a simple blinking-effect for my animations, and put this code into the eyes:

var timeDelay = 2000;
setFrameTimer = function(){
	if ( (Math.random(1)*100) > 90 ) {
setInterval (setFrameTimer, timeDelay);

Nice and easy, right, giving the eyes a 10 percent chance of blinking every two seconds.

Well, it works nicely in the beginning. But then, after a while, the blinking increases until finally the animations are blinking constantly.

Whyyyy? It just shouldnt happen, right?