code working only in FF and Opera

code working only in FF and Opera

every other browser fails. there's got to be a simple explanation for this. code:


$(document).ready(function() {
   
    $('.next').click(function() { nextSlide(); return false });
    $('.back').click(function() { prevSlide(); return false });
   
    setInterval ( 'nextSlide()'
        , 9000 // duration
    );

});


function nextSlide() {
    //alert('"NEXT" called');
   
    //  get the current 'left' position of the div
    oPosition = $('.items').css('left');
   
    //  strip the 'px' off of the value, for use in calculation
    posCalc = oPosition.indexOf("px");
   
    // current 'left' position, sans 'px' - parseInt turns the string into an integer
    rPosition = parseInt(oPosition.substr(0,posCalc),10);
   
    // new 'left' position is the current 'left' position plus 677px (width of image)
    //mValue = rPosition + 677;
    // mValue = -mValue;
    //alert('Real position is '+rPosition);
   
    if(rPosition == -2708){
        newLeft = 0;
    } else {
        newLeft = (-rPosition)+677;
    }
   
    //alert('New position is '+newLeft);
   
    $(".items").animate({
    left: -newLeft
    }, "slow" );
                  
    return false;   
}



function prevSlide() {
    //alert('"NEXT" called');
   
    //  get the current 'left' position of the div
    oPosition = $('.items').css('left');
   
    //  strip the 'px' off of the value, for use in calculation
    posCalc = oPosition.indexOf("px");
   
    // current 'left' position, sans 'px' - parseInt turns the string into an integer
    rPosition = parseInt(oPosition.substr(0,posCalc),10);
   
    // new 'left' position is the current 'left' position plus 677px (width of image)
    //mValue = rPosition + 677;
    // mValue = -mValue;
    //alert('Real position is '+rPosition);
   
    if(rPosition == 0){
        newLeft = 2708;
    } else {
        newLeft = (-rPosition)-677;
    }
   
    //alert('New position is '+newLeft);
   
    $(".items").animate({
    left: -newLeft
    }, "slow" );
                  
    return false;   
}