Bugzilla random lag and CPU usage

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.

From serverfault Serty Oan

• Check your network interfaces for errors and retransmit counts. It you are dropping packets you may have to wait for timeouts before the packet is retransmitted. Any load graphs for the network would help as well.

  Serty Oan : Thing is that only the Bugzilla app is slow randomly, other services (in PHP) do not suffer any slowness
  From BillThor