Can’t paste more than a few lines of text

I was creating pages and copy/pasting my text into each one just fine until I came to the last page. Now when I copy/paste, I get this error:

“Forbidden
You don’t have permission to access /wp-admin/page.php on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request. “

I double-checked all my permissions using this guide:
http://wordpress.org/support/topic/50863?replies=15

I’m able to copy/paste a few lines of text without getting the error message but anything more than that generates the error. Same message whether I paste it through MS Word paste, plaintext paste, or just paste it first into notepad to remove formatting and then paste it into the text box.

I tried changing the page.php permission to 7, 7, 7 just as a test but still got the same error.