First post full width, rest in two columns

you could rewrite the full code and use the build-in loop counter $wp_query->current_post to fix the css classses and add a class for the first post per row to clear the float to prevent the ‘wacky sticking’;

example:

<?php if ( have_posts() ) : ?>

<?php while ( have_posts() ) : the_post();  ?>
        <?php /* Start the Loop */ ?>

        <div class="entry-content-<?php echo (( $wp_query->current_post == 0 ) ? 'main' : 'rest' ); if( $wp_query->current_post%2 == 1 ) echo ' left-post'; ?>">
        <?php get_template_part( 'content', get_post_format() ); ?>
        </div>

<?php endwhile; ?>

additional css:

.left-post { clear: left; }