S~
Have you tried clearing your browsers' caches? If the index file in your cache is corrupt it'll keep happening until the index.php reaches its expiration date and your browser replaces it.
It seems your browsers are downloading the php instead of opening it. Usually caused by something whacky in the php or html for the site, but can also be caused by corrupt cached files, and yep, compression errors.. I vaguely recall a performance setting maybe in network, or maybe the browser's options? wherever it was, unchecking performance would fix compression problems sometimes, too.