Unbound doubles memory usage after some time

Hello Unbound Users,

we have unbound version 1.4.5 (linked libs: event 3.9, ldns 1.4.0,
OpenSSL 0.9.8o 01 Jun 2010) compiled with " --without-pthreads
--without-solaris-threads" running on Debian Lenny (kernel
2.6.26-2-amd64) and some times observe strange memory behaviour: after a
few days where unbound runs quite smooth, it suddenly uses approximately
twice the amount of RAM than before. Any hints on this issue?

Our config file looks like this: