Previous / Next button moving slightly on click

Part of it is normal. The default style for a button element is to move slightly on the click event to emulate physically depressing a button.. to give the user some kind of feedback. That's minor and intended behavior in the browser.

What you're probably seeing is the effect of the ajax spinner graphic loading and pushing the button element down a bit. You can do something like this to absolutely position the spinner graphic so it doesn't negatively impact the button placement.

[css]
/* make the page footer relative for positoning */

body .gform_wrapper .gform_page_footer {
    position: relative;
}

/* position the ajax spinner graphic absolutely inside the page footer */

body .gform_wrapper .gform_page_footer .gform_ajax_spinner {
    position: absolute;
    top: 25px;
    left: 100px;
    border: none !important;
}

my test: http://screencast.com/t/VbMHZYGpODE