[Theme Diamond] Post thumbnails overlap randomly?

Hello i am making my website using wordpress theme “diamond”, but have come to a problem regarding the post thumbnails at the front page.

The thumbnails overlap each other randomly and sometimes they look perfect when i refresh or choose to + zoom 110% and then go back to 100%.

It occurs mostly on Safari but also sometimes in Chrome and Firefox.

Here is the frontpage:
http://matylda.dk

post-thumbnail (diamond/style.css)

.hentry .post-thumbnail {
  display: block;
  overflow: hidden;
  position: relative;
  clear: both;
  z-index: 10;
  padding: 15px;
  border-bottom: 1px solid #ededed;

}

Hope you can help me out or guide me to the root of the problem.