Posts aren’t breaking properly in IE8

We’ve recently added a custom theme to our WordPress Blog, to fit the company’s branding. We first changed the theme on an offline WAMP host, and on that local host, everything is fine, no bugs, no glitches, everything works perfectly in every possible browser, even in IE8.

Yesterday, we’ve uploaded the new theme to our online blog, initialy, everything seemed fine here as well.. Until we loaded up the page in IE8.

This is what we found: http://i49.tinypic.com/20z8cr5.jpg

To sum the problem up:

1. The first/top post shows up just fine, there’s linebreaking and everything fits well
2. Every post following that first one, doesn’t have any linebreaking anymore.
3. If we refresh the page a few times, there’s a chance all the posts look fine, but the next refresh it’ll be broken again
4. This doesn’t happen at all on an offline WAMP server in IE8
5. This only happens in IE (8, though I can’t test it in older version at the moment)

Is there anything I’m missing here? Is this a common problem?

Detail: the line where the text cuts off, is the line of the original sidebar, just turned white, so I guess the text just goes right behind it, because the div of the post doesn’t limit it’s width, despite it having a fixed width in CSS.