help with css structure

First, sorry for my english it’s not my native.

I’m modifying kubrick theme. I would like to have just one post per ‘page’. I achieve this easy, but now as you can see here, when you walk through the post the whole page is moving left or right by round 5px. This doesn’t happen when I change
this:
#page {
background-color: white;
margin: 0px auto;
padding: 0;
width: 760px;
border: 1px solid #959596;
}

to this:
#page {
background-color: white;
margin: 0px 10px;
padding: 0;
width: 760px;
border: 1px solid #959596;
}

in style.css. 10px is just an example. I’m using firefox.

Any suggestion?