Here is the configuration :
Bugzilla (3.4.2) application running on a Ubuntu server 8.04 with Perl 5.8.8.

Here is the problem :
Sometimes (randomly), pages take very long to load. It can be any of the login page or query.cgi page or buglist.cgi... etc...
Using top on the server, I tried to see what was wrong, and saw that sometimes a bugzilla script would use 30% memory and last pretty long (1 to 5 seconds) and sometimes not even show in the list because it responded too quickly or just use 2%. The mysqld process does not seems to be the problem.