CSS Style Seems to Depend on Post Number

On my blog, the CSS styling works for all posts starting with the current post back to about 55 previous posts, when displayed as single pages. From that point on back, the styling is messed up. Here are 2 examples:

http://laceylibertarian.us/?p=1592 (looks OK)

http://laceylibertarian.us/?p=1533 (styling fails)

When not displayed as single pages, everything looks OK. But many people come to my blog using the single page URI and it looks bad. And, no, I haven’t changed anything during the time period between those posts. Also, same odd behavior in IE and Firefox.

I’d appreciate any help. Thanks.