Issue with theme not appearing

Hi,

I am having an issue in which my theme sometimes "disappears" for lack of better terminology. It is not always the same post, but the header and all the theme elements will just be gone and then later they return. It doesn't happen to the whole site at once, just individual posts. I took a screen shot, but I don't have a way to include it in this note. I would really appreciate any help. Thank you.