Problem med action på radiobuttons

Hvorfor skal jeg klikke 2 gange på hver radiobutton før jeg bliver kastet videre?

<table width="30%" border="0" align="center">
  <tr>
    <td>
      <form action="" method="post" name="form">
  <h3>Sorter efter..</h3>
  <input type="radio" name="radiobutton1" value="radiobutton"  onClick="MM_goToURL('parent.frames[\'mainFrame\']','forum2.php?Sort=senesteId');return document.MM_returnValue">
 
  <input type="radio" name="radiobutton2" value="radiobutton" onClick="MM_goToURL('parent.frames[\'mainFrame\']','forum2.php?Sort=datoId');return document.MM_returnValue">
 
  <input type="radio" name="radiobutton3" value="radiobutton" onClick="MM_goToURL('parent.frames[\'mainFrame\']','forum2.php?Sort=authorId');return document.MM_returnValue">
 
  <input type="radio" name="radiobutton4" value="radiobutton" onClick="MM_goToURL('parent.frames[\'mainFrame\']','forum2.php?Sort=overskriftId');return document.MM_returnValue">
 
<?php
    if ($SortForum == "senesteId") { ?>
        <script>document.form.radiobutton1.checked = true; </script><?php }
    else if ($SortForum == "datoId") { ?>
        <script>document.form.radiobutton2.checked = true; </script><?php }
    else if ($SortForum == "authorId") { ?>
        <script>document.form.radiobutton3.checked = true; </script><?php }
    else if ($SortForum == "overskriftId") { ?>
        <script>document.form.radiobutton4.checked = true; </script><?php }
    else { ?><script>document.form.radiobutton1.checked = true; </script><?php }
        ?>
</form>
</td>
  </tr>
</table>