Rails action using an insane amount of MySQL/CPU

I have a very large rails app running on 4 instances of mongrel on a P4
server with 1GB RAM. Not the absolute best setup, but, the server has
been optimized and the application has been running extremely fast for
the past few months.

I'm having one serious problem however...Theres a specific action that
uses 99% of MySQL on the server and in most cases, doesn't even load, I
end up with a "520 Proxy Error, Reason: Error reading from remote
server" error from mongrel.

The action in question really is like any other, It grabs images tagged
with a specific tag and paginates them:

@image_pages, @images = paginate :images,
                                    :include => "tags",
                                    :conditions => (["tags.name = ?",
                                    :order_by => "images.created_at
                                    :per_page => 12

I can't figure out what's going on, it seems to run fine on my
localhost, but simply kills the production server. The rails log looks
normal, the query is very large after the associations, but I have
similar queries that run just fine.

How could I go about figuring out what the problem is?

Any help would be extremely appreciated!

How many images/tags are in the database? paginate is not very efficient when it comes to large datasets as it grabs them all (at least that's my memory).

I'd take a look at the dev log and the SQL being generated and see what it is that's going on. Perhaps pass those queries into mysql prefixed with "EXPLAIN " to see if it's using your indexes or not...