I think the problem is caused by raytraced shadows. I also have seen/experienced the dreaded ink-blot of doomed firefly rendering. I seem to remember that someone advised me to increase the shadow bias. Personally, I found that tinkering with the lights was such a PITA that I mostly only use Poser for the posing and POV-Ray for the final rendering (of static scenes). I will also be watching the responses to see if there if a definitive solution.