Try giving your whole site a container with a common background and 100% width. example:
The only issue I saw was the background of all your divs kept being cut off. Those divs were showing their backgrounds based off the window size. However, if contained within a containing div they will fill out the container instead.
also, I played with your code. you need to give elements like your header and main_content a width. I got better results that way.
do both of those and you should see an improvement. Let me know how it goes.