Everything in the site works fine, but when I want to create or modify a content item the problem begins. My Joomla slows down a lot, loading forever and ever, nothing happens, sometimes FF opens up a download window in order to save index2.php in my PC, very weird

(check attachment image)
I do not think is the Editor (XHTML Suite 1.8.x) because this happens even with no editor configured.
How can Identify what's wrong? Apache? MySQL? the server itself? Joomla user I use?
I know Joomla is not the headache, cause I have many sites in the same shared hosting with the same extensions and this is the only one I have a very poor performance when creating/modifying content items. The only different I notice with the rest of the sites is the size, the site is bigger than other, about 2.1 GB in file system. The compressed SQL gzipped backup file is 10 MB (created by phpMyAdmin).
Sometimes I look into error logs but without lucky, I only find common "File does not exist" errors on jpg's or gif's, not find anything with importance.
my system info:
Joomla 1.0.15
Database Version: 4.1.22-standard
PHP Version: 4.4.7
Web Server: WebServerX
WebServer to PHP interface: apache
Joomla! Version: Joomla! 1.0.15 Stable [ Daytime ] 22 February 2008 23:00 UTC
User Agent: Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US; rv:1.8.1.14) Gecko/20080404 Firefox/2.0.0.14
I really would appreciate any suggestion on how to identify what's wrong...
Thanks everybody
