Cycles Render. I have a simple scene, with a plane as ground, and objects on it. It is lit by a HDR background texture and one sun lamp, and being rendered to 16-bit TIFF individual frames. The ground plane color begins to flicker a darker version of the same hue after about 240 frames, and then eventually goes to the darker color after a dozen or so flickers over the next 120 frames. The ground material is simple Diffuse BSDF ... What is causing this flicker?
See flickering animation at: https://youtu.be/QSf4CFDx6Zk