Very long delay when publishing/editing a post

I am running wordpress 2.5.1 from July 2008 on my dedicated server.
Since 24th october 2008 my blog become very very slow and sometimes inaccessible. I don’t know why, I didn’t add any new plugin since September .. I have an extremly high load on my server, normally I use 300 Mb/1024Mb of my server Ram .. now I am using more than 900 Mb.
I tried to disable all plugins and disable pinging to services .. the problem remain the same.
I tried to use a backup of my database a 23rd october one, just edited WP-Config to run under this backup ..
After that everything became to normal : my blog is fast and my Ram usage is normal .. thought I finally resolved the problem but when I edit an existing post or create a new one I have to wait 4 minutes to get my post published and from the moment I click on the publish button the blog become more and more slower and the Ram usage begin jumpin from 250 Mb to 900 Mb.

I don’t what to do .. anyone have an idea ?