var g_iCount = 72;

function startCountdown() {

   if ((g_iCount - 1) >= 11){
           g_iCount = g_iCount - 1;
           document.getElementById('countdown').innerHTML = g_iCount;
           
           randomnumber = 6000 + Math.floor(Math.random()*4001)
           
           setTimeout('startCountdown()',randomnumber);
   }

}