After a few minutes running some files are processed extremely slow (more than 30 sek) on my lighttpd server.
A file like
test.php
takes more than 30 sek.

while a file like test.txt
takes 0 sek

Also
A image located at
example.com/blog/files/2010/01/picture.jpg
takes 30 sek to load

while the same image located at
example.com/blog/wp-content/blogs.dir/1/files/2010/01/picture.jpg
takes 0 sek to load

The address on the filesystem for the images is
/var/www/example.com/public/blog/wp-content/blogs.dir/1/files/2010/01

The system is a debian lenny on 2 cpu, 4 cores, 8 Gb MEM and is idle most of the time.