Currently the SRC_URI for the newrelic-sysmond ebuild points to a volatile location only containing the latest release. Instead it should use the archive directory: SRC_URI="http://download.newrelic.com/server_monitor/archive/${PV}/${P}-linux.tar.gz" which also contains the current release to avoid broken downloads.
Resolved in 14cbab6846752d6e0a766fa824482c535ed50da3. This package is currently without a maintainer. Please consider adopting this package through the Proxy-Maintainers project (https://wiki.gentoo.org/wiki/Project:Proxy_Maintainers)