Can't upgrade or update plugins: "could not create directory"

Hi, I’m working around a wordpress installation for the last days to solve this :
Every time I try to upgrade WP or update a plugin I get a message Could not create directory.: /home/.../public_html/.../wp-content/upgrade/wordpress-3.tmp/wordpress , I deleted the folder and created a new one manually giving 777 on permissions, but the same message comes up. The only way to add or update a plugin is by loading via ftp to plugins directory.
Any help please?

(version 3.0.5)