Wrong loop output

Hi, I’m working on a WordPress with TwentyTwelve theme installed.
I’ve problems at this page:
http://iam.altervista.org/archalp/

It should show all the thumbnails and titles of the posts of a category.

PROBLEMS:
1 – Output in the wrong order. I don’t know why it starts from the center on the second row, and it doesn’t follow order by “date” or “number” atttribute.
2 – The titles above the images sometimes are incomplete, infact not all of them shows the number of the image and its date. However if I open the post, the title is complete, so no information is missing from the database.

CODE:

<header class="entry-header">
<?php if ( ! post_password_required() && ! is_attachment() ) :
	the_post_thumbnail();
endif; ?>

<?php if ( is_single() ) : ?>
	<h1 class="entry-title"><?php the_title(); ?></h1>
	<?php else : ?>
	<h1 class="entry-title">
		<a href="<?php the_permalink(); ?>" rel="bookmark"><?php the_title(); ?></a>
	</h1>
	<?php echo get_post_meta($post->ID, 'sottotitolo_archalp', true); ?>
	</header><!-- .entry-header -->

	<?php
		$paged = ( get_query_var( 'paged' ) ) ? absint( get_query_var( 'paged' ) ) : 1;
		$args = array( 'category_name' => 'archalp', 'posts_per_page' => 8, 'orderby' => 'numero_della_rivista', 'order' => 'DESC', 'paged' => $paged );
		$loop = new WP_Query( $args );

		the_content( __( 'Continue reading <span class="meta-nav">→</span>', 'twentytwelve' ) );

		echo '<br /><br /><br />';

		?>
		<div class="elenco_archalp"><ul>
			<?php while($loop->have_posts()) : $loop->the_post(); ?>

			<li>
			<div <?php post_class(); ?> id="post-<?php the_ID(); ?>">
			<a href="<?php the_permalink(); ?>" />
				<h1>
					<?php the_date( 'm,Y', the_title() . ' n.' . get_field('numero_della_rivista', $loop->ID) .' - ', '', TRUE ); ?>
				</h1>

				<?php the_post_thumbnail('archalp-thumb'); ?>

			</a></div></li>
	<?php 	endwhile;
				echo paginate_links( array(
					'total' => $loop->max_num_pages,
					'mid_size' => 2
				 ));
	?>
	</ul></div>

<?php wp_reset_postdata(); // reset the query ?>
<?php endif; // is_single() ?>

Thnaks in advance!