Archived and Categories not being paginated correctly

Here’s my site: http://tambnguyen.com/, and I’m using WP 3.1 with WP-PageNavi 2.74.

My problem is when view the site under either Categories or Archives, the pagination is not working correctly. For example, in Feb 2011, I posted 12 entries. When viewing http://tambnguyen.com/2011/02/, it only shows the first page. When clicking on the second page (http://tambnguyen.com/2011/02/page/2/), I get a soft 404. The same thing happens when viewing the categories.

I’ve tried disabling all the plugins to the bare minimum, and yet still have that problem. Currently, I have DB Cache Reloaded, W3 Total Cache (Minify, Object cache, Browser cache and CDN enabled).

Here’s my index.php:

<?php if (is_archive()) $post_number = get_option('polished_archivenum_posts');
if (is_search()) $post_number = get_option('polished_searchnum_posts');
if (is_tag()) $post_number = get_option('polished_tagnum_posts'); ?>
<?php get_header(); ?>

	<?php global $query_string; query_posts($query_string . "&showposts=$post_number&paged=$paged"); ?>

	<div id="wrap">
	<!-- Main Content-->
		<img src="<?php bloginfo('template_directory');?>/images/content-top.gif" alt="content top" class="content-wrap" />
		<div id="content">
			<!-- Start Main Window -->
			<div id="main">

				<?php if (have_posts()) : while (have_posts()) : the_post(); ?>

					<?php include(TEMPLATEPATH . '/includes/entry.php'); ?>

				<?php endwhile; ?>

					<?php if(function_exists('wp_pagenavi')) { wp_pagenavi(); }
					else { ?>
						<?php include(TEMPLATEPATH . '/includes/navigation.php'); ?>
					<?php } ?>

				<?php else : ?>
					<?php include(TEMPLATEPATH . '/includes/no-results.php'); ?>
				<?php endif; wp_reset_query(); ?>
			</div>
			<!-- End Main -->
<?php get_sidebar(); ?>
<?php get_footer(); ?>

and here’s my category.php

<?php get_header(); ?>

	<div id="wrap">
	<!-- Main Content-->
		<img src="<?php bloginfo('template_directory');?>/images/content-top.gif" alt="content top" class="content-wrap" />
		<div id="content">
			<!-- Start Main Window -->
			<div id="main">
				<?php global $query_string; $catnum_posts = get_option('polished_catnum_posts') ;
			query_posts($query_string . "&showposts=$catnum_posts&paged=$paged&cat=$cat"); ?>
				<?php if (have_posts()) : while (have_posts()) : the_post(); ?>

					<?php include(TEMPLATEPATH . '/includes/entry.php'); ?>

				<?php endwhile; ?>

					<?php if(function_exists('wp_pagenavi')) { wp_pagenavi(); }
					else { ?>
						<?php include(TEMPLATEPATH . '/includes/navigation.php'); ?>
					<?php } ?>

				<?php else : ?>
					<?php include(TEMPLATEPATH . '/includes/no-results.php'); ?>
				<?php endif; wp_reset_query(); ?>
			</div>
			<!-- End Main -->

	<?php get_sidebar(); ?>

<?php get_footer(); ?>

Please take a look and thanks for your help guys.