Single.php menu hightlight problem

Hi, I have several Pages, the Menu is create from Page, not Category.

I included <?php wp_nav_menu(array(‘theme_location’ => ‘main_menu’)); ?>
in header.php, this is the menu css

#main_menu {
	font: 62.5%/1.3 'trebuchet ms',verdana,'Lucida Grande',arial,sans-serif;
	background-color: #fff;
	width: 960px;
	z-index: 1000;
}
#main_menu ul {
	list-style: none none;
}
#main_menu ul li {
	float: right;
	position: relative;
	/*display: block;*/
	/*color: #fff;*/
	margin-left: 5px;
}
#main_menu ul li a {
	width: 99px;
	text-align: center;
	display: block;
	background-color: #888;
	color: #ccc;
	font-size: 11px;
	line-height: 21px;
}
#main_menu ul li a:hover,
#main_menu ul li.current_category_ancestor a,
#main_menu ul li.current_cat a,
#main_menu ul li.current-menu-item a,
#main_menu ul li.current-menu-parent a,
#main_menu ul li.current_page_item a,
#main_menu ul li.current_page_ancestor a,
#main_menu ul li.current_page_parent a,
#main_menu ul li.current_post_ancestor a,
#main_menu ul li.current_post_parent a,
#main_menu ul li:hover > a {
	width: 99px;
	text-align: center;
	background-color: #666;
	color: #fff;
}

My pages can render the menu without issue, the menu item highlight properly when switch from page to page. Inside the page I have a loop to list all post title. I have single.php for display single post.

Here is the output from page.php

<li id="menu-item-219" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-219">
<li id="menu-item-220" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-220">
<li id="menu-item-221" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-221">
<li id="menu-item-222" class="menu-item menu-item-type-post_type menu-item-object-page current-menu-item page_item page-item-169 current_page_item menu-item-222">
<li id="menu-item-223" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-223">

My problem occur when I click on the post title, the single.php can show my content and the menu, but the menu item is not highlight.

I inspect the output and find the menu items does not have any css associate to highlight:

<li id="menu-item-219" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-219">
<li id="menu-item-220" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-220">
<li id="menu-item-221" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-221">
<li id="menu-item-222" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-222">
<li id="menu-item-223" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-223">

So what is my problem, I tackle with this issue for long but didn’t found solution or workaround.

Any help is greatly appreciated. Thank you.