Hi Craig,
Unless I'm missing something, your page seems to be breaking down around the 768px marker. I measured the breakpoint using the Chrome MeasureIt extension and it doesn't appear to be triggering the next media query at 854px. Its triggered at a lower px dimension. I also checked out your page on my tablet and smartphone and all seems to look as expected.
But then again, I could be completely misunderstanding your problem.
Mike