First, meticulously go over your HTML to be certain that your
class and id assignments are consistent between pages.
That is the #1 reason why page alignments tend to shift.
Second, your CSS seems to be diversely scattered among a number
of style sheets. You need to integrate your styles into a single
document, if possible. This makes tracing class and id mismatches
and duplications very much easier.
Your main images are way too big at 300-400kB. Not everyone has
8Mbps+ broadband ! Use 'Save for Web and Devices' in Photoshop. You will
be surprised at how little detail is lost and the files will be 1/10th the size.