Disabling back button for specified time

Hi,

I used to use the code below to disable the 'Next' button for a specified time. It replaced the next text with a countdown timer. With this newer software this no longer seems to work. Can anyone tell me where this broke? I assume it is simply a reference to how the next button is called.

<script type="text/javascript">

function disableNext ( )
{
  originalValue = document.mainform.sys_next_button.value;
  document.mainform.sys_next_button.disabled = true;
  document.mainform.sys_next_button.value = "Please wait (" + timeOutRemaining-- + " seconds)";
  setTimeout ( "countDown()", 1000 );
}

function countDown() {
	document.mainform.sys_next_button.value = "Please wait (" + timeOutRemaining + " seconds)";
	if(timeOutRemaining > 0) {
              timeOutRemaining--;
              setTimeout("countDown();", 1000);
        } else {
    	      enableNextButton();
        }
}

function enableNextButton ()
{
  document.mainform.sys_next_button.disabled = false;
  document.mainform.sys_next_button.value = originalValue;
  
}

window.onload = disableNext;
var originalValue;
var timeOutRemaining = 30; //30 Seconds
  
  
</script>