First things first, we have 40 errors showing up right now in the HTML:
The majority of these appear to be due to using certain IDs more than once in a page. Change these items to classes instead of IDs (and remember to change the "#" to a "." in the CSS for each). The rest of the errors look to be some issues with the footer's menu. I'll let you sort those out.
Now, from what I can see you have only added
to "portfolio-item-box" which would be the cause of the problem (margin collapsing as you mentioned earlier). Change that to
and you should be all set.
Also, you have a bit of "divitis" in there. For example, using a div to wrap an image for your portolio items is totally unnecessary. You might want to streamline things like that a bit.