Differences in CSS between pages.

This is typical IE behavior. IE always seems to need more width for elements than other browsers. You may, if you like, feed IE a different dimension through the use of the star html hack, anything after which only IE sees. To ensure it will last into the future through the next IE upgrade, feed it in through a conditional comment in the head of your header.php file (instead of your stylesheet) like so:

<!--[if lte IE 6]>
<style>
* html .yourclassordivhere {
width: IEpx;
}
</style>

<![endif]-->

Do check and see if this now plays nicely in IE5 and IEMac since they act differently than IE6.