Category Archive page is breaking

Hi – something funky is happening on my category archive page. I stripped down the code so it just displays the name of the category, and then links to each post in the category. but for some reason, after the third post listing, the page div closes and the formatting goes crazy. here’s the category.php code at pastebin and here’s a screenshot of what it looks like

(I would just paste a link but the page is live and password protected for students.

any ideas?

thanks!