layout broken

hi there,
the archive page i have trouble with originally showed a column of posts: each with the post image and the post title and with the same gap between the posts. now i realized that the gaps beetween the posts vary randomly: either the space is too big or it disappears completely so that the post title is covered by the following post image.
the post position is set in an element style to “absolute” and a certain pixel value for “top” that seems to be calculated somewhere. can anybody help my to find out where the gap size between the posts is defined so i can set all gaps to the same height?

The page I need help with: [log in to see the link]