setInterval / clearInterval driving me crazy!

this conditional i have in this function works… everything traces out… .for some reason it won’t clear the setInterval!!! arrrgh!!!

_global.cTimer=function(tc)
{	
	trace("cTimer Function: "+tc);
	if(tc=="start"){
		if(sI != null){
			clearInterval(sI);
		}
		var sI:Number;
		sI=setInterval(dayOut, fadeTime);
	}else{
		trace("CTIMER FUNCTION STOP!");
		clearInterval(sI);
	}
}

‘dayOut’ is a _global function. what am i doing wrong???

also, when clearing an interval, does the interval id return to null? or do i have to manually set that?