lucy24 - 11:33 pm on Dec 13, 2011 (gmt 0)
By "formatting dies" do you mean that halfway through the page, it stops seeing your CSS? I'm impressed. I can do that easily for the whole page, but mid-page is harder.
The first place to look is the exact location where the formatting stops. And then start looking backward. That is, if your error happens in the middle of a series of <p class = "foobar"> then the problem may not become visible until the next time your html has to consult the CSS for something new.
Most obvious thing to look for: Are you using relative links to call your CSS? If so, does the page use the contents of more than one directory?