Uneven height - problem with column

[ZackWhite]

It are 8 posts and they should be ordered like this:
1234
5678

Now they are:
1234
xxx5
678

They don't have to be the same height, they just have to be aligned next to each other (each row = 4 articles) without that big empty hole in the middle. But I have no idea why this happens.

And yes, I'm using skeleton.

EDIT:
- Removed all the custom CSS and still the same problem.