Slow forum, low server load, many mysql sleep commnads

[KForum]

Hi!
We have a rather akward situation that arises from time to time: some pages of our forum are not loading correctly.
The browser says "waiting for server" and just hangs for minutes.
Meanwhile, the server load is low (0, 25), memory usage is normal, but we have have a lot of mysql sleep connections (60-70).
The strange thing is that while a certain url is taking forever to load, other url-s work perfectly.

We use vbulletin 4.1.12 with vbseo, mysqli connector and memcached.
PHP 5.3.17,

Server version: Apache/2.2.22 (Unix)
Server built: Mar 3 2012 19:30:34

mysql --version
mysql Ver 14.14 Distrib 5.1.61, for unknown-linux-gnu (x86_64) using readline 5.1

The server is Debian 6 with DirectAdmin, 5 cores, 7 Gb RAM.

Does anybody have any clues to what might cause this problem?
If you need some sort of logs, I can post them.

Thank you!