ok i find the solution ...
in itemlist.php located in com_k2/models ( verison 2.6.8 DEV BUILD )
replace at line 330 the case 'rand' with that :
case 'rand':
$currentSession =& JFactory::getSession();
$sessionNum = substr(preg_replace('/[^0-9]/i','',$currentSession->getId()),2,3);
$orderby = 'RAND('.$sessionNum.')';
break;