Linux – How to Find the Cause of Growing Server Load

I'm having load problems with my server and even though I'm a somewhat experienced Linux admin I'm out of ideas now.

The problem is a slowly but steadily increasing load on the server without any apparent cause.

The Server is a AMD Athlon(tm) 64 X2 Dual Core Processor 6000+ with 6GB RAM. It is running Debian Stable with Linux gir 2.6.26-2-amd64 #1 SMP Wed Aug 19 22:33:18 UTC 2009 x86_64 GNU/Linux.

The server basically runs Lighttpd, several FastCGI PHP processes and a MySQL database. Typical webserver tasks.

The CPU is never really fully used up and memory is mainly used for buffers and cache which is fine. I tried to restart the various services to see if one of them would decrease the load again, but without luck.

Here are graphics showing load, CPU and IOStat:

So, question is: What could cause a slowly but ever increasing load? And how do I find out what's responsible?

Update: I forgot to mention, when I reboot the server, the load will be down to around 0.3 to 0.6 and will start to climb up again slowly over the next weeks.