There is a bug like feature going on with pages that lists posts. The space between the heading and the content of the posts listed will vary from each time the page loads. Sometimes the space can be more than 600px or so. Also the space from one post till the next will vary like this.
Mac OS 10.6.8, Firfox 14.0.1, Google Chrome and Safari also tested with the same result. Tested both as logged in and not logged out. I turned off the maintenance plugin to see if that was the cause, but no such luck.