Having issue with position of jQuery Dialog Box “Center "

Having issue with position of jQuery Dialog Box “Center "

Hi Team ,

I have an issue with  position jQuery Dialog Box “Cente" , first time I am getting at center but from 2 onwards the position of dailog box is not consistent.
PLease suggest me the code  so that dailog box is in center. please suggest me
I have written the code as
 $( "#basicModal" ).dialog({
         title: "Are you sure?",
         minWidth: 400,
         height: "auto",
         dialogClass: "no-close,dlgfixed",
         resizable: false,
         position: { my: 'top', at: 'top+150' }
  }).css("font-size", "0.8em");