Customer walker for creating JumpMenu

Hi, I’m trying to dynamically populate a JumpMenu using code I’ve found for a custom walker:

functions.php

class Walker_Nav_Menu_Dropdown extends Walker_Nav_Menu{

    // don't output children opening tag (<code><ul></code>)
    public function start_lvl(&$output, $depth){}

    // don't output children closing tag
    public function end_lvl(&$output, $depth){}

    public function start_el(&$output, $item, $depth, $args){

      // add spacing to the title based on the current depth
      $item->title = str_repeat("&nbsp;", $depth * 4) . $item->title;

      // call the prototype and replace the <li> tag
      // from the generated markup...
      parent::start_el(&$output, $item, $depth, $args);
      $output = str_replace('<li', '<option', $output);
    }

    // replace closing </li> with the closing option tag
    public function end_el(&$output, $item, $depth){
      $output .= "</option>\n";
    }
}

In my theme:

<form name="form" id="form">
  				<select name="jumpMenu" id="jumpMenu1" onchange="MM_jumpMenu('parent',this,0)">
                	<option selected="selected">Choose a team</option>
    				<?php wp_nav_menu(array(
  						'theme_location' => 'teams', // your theme location here
  						'walker'         => new Walker_Nav_Menu_Dropdown(),
  						'items_wrap'     => '<option>%3$s</option>',
						)); ?>
  				</select>
		</form>

This works on the homepage but not when you drill down to another sub-menu where the same menu appears. Also it won’t output the slug only the menu title. Is there any tweaks I can make to the code to make this happen?

Thanks

Darren