Key listening in Flash Lite 2 unexpected results

I'm trying to display a text when a key is pressed at regular interval like 500, half a second. At the start, if I press a button like Up, the screen would display Up every half a second, the other half of a second there is no text on the screen, which is what I intended to do. But when I pressed other key, the interval mess up and also the text on the screen. I couldn't find what is wrong. This is the code I'm working with:

Actionscript Code:

this.createTextField("trace_txt",1,0,0,200,200);
trace_txt.multiline = true;
trace_txt.wordWrap = true;
var txtFormat:TextFormat = new TextFormat();
txtFormat.color = 0xFFFFFF;
txtFormat.bold = true;
trace_txt.setNewTextFormat(txtFormat);

var traceOn:Boolean;

function blink(arg)
{
    if(!traceOn)
    {
        trace_txt.text = arg;
        traceOn = true;
    }
    else
    {
        trace_txt.text = "";
        traceOn = false;
    }
}

var keyListener:Object = new Object();
keyListener.onKeyDown = function()
{
    if(Key.getCode() == Key.DOWN)
    {
        setInterval(blink,500,"Down");
    }
    
    if(Key.getCode() == Key.UP)
    {
        setInterval(blink,500,"Up");
    }
    
    if(Key.getCode() == Key.LEFT)
    {
        setInterval(blink,500,"Left");
    }
    
    if(Key.getCode() == Key.RIGHT)
    {
        setInterval(blink,500,"Right");
    }
}

Key.addListener(keyListener);

EDIT: I have tried using the SWF in Device Central and my phone. Both had the same result and the text interval is messed up when another button is pressed.