sidebar, footer problems with theme

I’m having two problems with my theme:

1. The sidebars disappear randomly, then reappear when the browser is refreshed.

2. I can’t get the footer to appear all the way at the bottom of the page, below the longest bit of content (2 sidebars and main content).

What am I doing wrong?

Also, I’m unable to edit my theme using theme-editor.php. I think the problem might be related to this, but I don’t know how to fix it. Here’s my stylesheet:

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