If Linux is involved, you may have a version of selinux that leaks memory (disable selinux and reboot). Also, 389-ds 1.2.8 something may leak memory if the /var/lib*/dirsrv/*/db/userRoot/id2entry.db4 file exceeds the nsslapd-cachememsize setting.
https://bugzilla.redhat.com/show_bug.cgi?id=695440
https://bugzilla.redhat.com/show_bug.cgi?id=697701
http://docs.redhat.com/docs/en-US/Red_Hat_Directory_Server/8.2/html-single/A...
Also consider periodic preemptive graceful restarts at as appropriate times as possible to avoid hard crashes, if the leaks cannot be worked around.
Jeremy