Server :: Server Slows And Timed Out ?

I have a box running, within this box I serve approx 18 sites. Starting few months ago. the memory usage seems to climbs up everyday. I've check my cron, and I don't have any cron that runs everyday (but hourly cron that only did checking external server's availability via wget). The only new thing that I've add was a backup script to dump mysql database, which occurs weekly. (I can post it's script if needed).

Here is my weekly memory chart [url]

And lately things are getting worse, everytime I did this:

Code:

At least one of my site returns with timeout. When I visit the site using my browser, the site is actually reachable but it takes about 20 seconds to load (my wget use timeout 10 seconds so it will surely returns with timeout). Each time one or two sites downs randomly. But about two-three small sites dominates - their chance to get slowed seems to be higher than the rest 15 sites.

Is there any way I can detect the problem? I really don't have any idea...

My box runs on CentOS release 5.4 (Final)