Hi Andy,
Since /galley is your least tall page, I'd suggest the shift you are seeing is the result of the scroll bar disappearing for that page. That makes a new window width for your page to center within. Reduce your browser height to confirm; maybe add some dummy space content below the index to make that page taller if it bothers you.
Not being able to get into that page to edit is a bigger problem worthy of a support ticket.
I am finishing a site - www.markburginphotography.com - but for some reason when one of the pages ( 'gallery') is selected the banner shifts slightly to the right...
It is using the same template as 2 other pages so it should at least be the same as those ones being as i simply uploaded tha banner image into the editor but...
Also, and more annoyingly, whenever I am logged in and click on 'gallery', which is an html page with only linked images on it SQ logs me out so that I cannot review any page specifics that may be causing this problem - though off hand I cannot imagine what they may be
Thanks for the help