One thing I notice looking at your code, is that everything inside your content div is enclosed within h1 (Heading 1) tags.
It is possible that this is throwing things out under certain conditions.
A few lines after your opening <body> tag:
and a few lines before your footer div:
These h1 tags should not be there. This may not be the reason, but I just know it's probably going to turn out to be some little thing like this!