Why does my WP site break each week like clockwork?

My site keeps going down each and ever single week. Sometimes I get 404 or 500 errors, sometimes I get errors saying that either my header.php or index.php files are missing. This has been happening every weekend for the past 4 or 5 weeks. I’ve already called my web host and they can’t tell me anything. I’m the only one with the password and access to my FTP, I change the password often and it’s VERY long with a variation of numbers and case-sensitive letters. It’s getting a little monotonous having to back up and re-upload my site each and every weekend, especially when I have to deal with emails from my advertisers asking why the site’s down.

I’ve just recently repaired my SQL database and changed my password yet again, but I’d just like to know if there could be something causing these errors that I could prevent.

Thanks in advance.