MAGENTO : ONEPAGE CHECKOUT SCROLLING ISSUE.

Here i found good fix of this issue.
Open opcheckout.js and find gotoSection function and replace this function as follow.

gotoSection: function(section)

{

    var sectionElement = $('opc-'+section);

    sectionElement.addClassName('allow');

    this.accordion.openSection('opc-'+section);

    this.reloadProgressBlock(section);

    jQuery("html, body").delay(10).animate({scrollTop: jQuery("#opc-"+section).offset().top }, 1000);

},