LDAP queries sometimes slow

I’ve got the problem that sporadic every so many minutes LDAP queries are very slow. In this case the UMC blocks (e.g. opening a user object or saving changes to it takes multiple seconds) and logins to external web services which use the LDAP as backend take many seconds to a minute, too. After a short while everything is ok again (that means any operation responds immediatelly) until after some minutes this weird behaviour starts all over.
As I don’t have any clue which component might cause this behaviour, could you point me to some places to start investigating?
Thanks a lot!