Gaps between post columns

I made a theme that’s based on twentyeleven, and i transformed the posts into 2 columns. The only problem is when one post is taller than another it leaves a big gap underneath and I’m not sure how to fix the problem.
here is my website. (and a working example of the problem)

Here is the relevant code:

http://pastebin.com/SqD050cF [index.php]
http://pastebin.com/eUKYinZf [style.css]

in the css the #columns:after is based on this clear fix hack.

any help would be appreciated