Here's another data point. The artifacts also appear under the following
conditions, tested with the default (16) worker threads.
POV-Ray 3.7.0.RC7 (icpc 13.1.0 @x86_64-unknown-linux-gnu)
Intel(R) Xeon(R) CPU E5-2680 0 @ 2.70GHz
uname -or
2.6.32-279.14.1.el6.x86_64 GNU/Linux
lsb_release -irc
Distributor ID: CentOS
Release: 6.3
Codename: Final