It's this that's causing the problem:
When the screen is resized to a relatively small width the negative left margin takes it right off the left of the screen. Try replacing this with:
You may also need to rethink your background image, because the "shellfire" text also comes out of line when you resize. There might be a way around this without changing the image, but I can't think of one!
margin:130px auto 0px;