Problems with Content Slider

Hi!

I'm new to Contao but got along with it so far. Howsoever, I do now have a problem with the article element "Content Slider".
First I put some pictures between the "envelope" elements, but they were shown below the other instead of next to it.
I then figured out that I just need to reduce the width. So far so good, but now I do have the problem, that at slide 4 or 5 you can still
see a part of the slide before. I hope understand what I mean, if not, here is a link to the page:

http://machine.square7.ch/index.php/historik.html

I tried the german community but didn't got an answer there. I already found some similiar threads but none of the solutions worked out.

Does anybody knows the problem and has a solution?

BTW, I'm using Contao 3.4

Thanks!

Lars