Server going down

[eluser]johnwbaxter[/eluser]
Yeah i'll second Colins comments.

Can you post some stats about how many hits you get, how many requests per second, basically what the load is. Are you doing lots of database writes or is it mostly reads? How is your database setup? Have you got a lot of tables or just a couple of big ones?
If you turn on profiling how much memory does CI say it is using?
What are the specs of your server(s)?

Codeigniter as a framework has a very small footprint so it shouldn't be an codeigniter issue. Are you loading only the components you need per controller or are you autoloading more than you need sitewide?

Have you tried benchmarking your site?

The more info about your setup the more we'll be able to help.