Memory Leak in Actionscript 3 Substring Handling

I'm running the very begninning of a program I'm going to use for a text-based adventure, and it seems to be working very well, but I've noticed a severe slowdown in my program. I've been using trace(System.totalMemory); to discover the problem and there is a very linear increase in memory, about 500b-1kb per frame.

{
addEventListener(Event.ENTER_FRAME, fl_EnterFrameHandler);
function fl_EnterFrameHandler(event: Event): void {
    //this block sets up the blinking cursor
    var_x = var_x + 1;
    if (var_x > 18) {
        var_x = 0;
        if (str_cursor == " ") {
            str_cursor = "█";
        } else {
            str_cursor = " ";
        }
    }
    //Checking to see if a picture needs to be drawn or not
    if (picture != "") {
        fulltext = (picture + "\n\n" + responce + "\n\n" + ">>>" + user_input + str_cursor);
    } else {
        fulltext = (responce + "\n\n" + ">>>" + user_input + str_cursor);
    }
    //Setting the interval of the draw string
    uInt = setInterval(writeIt, 1);
    function writeIt() {
        if (!printed) {
            //Finding the substring of the draw string and assignign to the text field
            fl_TF.text = fulltext.substring(0, count);
            count += count_speed;
            if (count > fulltext.length) {
                clearInterval(uInt);
                //Stoping the print loop
                printed = true;
                count = 0;
            }
        } else {
            user_input = inputField.text;
            fl_TextToDisplay = fulltext;
            fl_TF.text = fl_TextToDisplay;
        }
    }
    if (printed) {
        user_input = inputField.text;
        fl_TextToDisplay = fulltext;
        fl_TF.text = fl_TextToDisplay;
        clearInterval(uInt);
    }
}

I can't seem to find the leak, as I've traced all my variables involved to the console at different points.

You can find a screenshot of the program here: http://ift.tt/1HgXueN