Copied from VRayForum:-
We are using a VrayClipper object (a large rectangular box) to cut a hole in the side of a building so that we can see the inside. Inside, we have a ForestPro object containing lots of chair objects. There are a number of lights in the scene, including a VRaySun, but no GI. We are animating a camera and rendering out frames.
Playing back the frames, the shadows from the sun flick on and off randomly for the ForestPro objects. It seems this is for any computer on our network: in other words, it isn't just one or two problematic machines. In fact, rendering the same frame locally will sometimes show the shadows correctly, and other times wrongly.
I'm not entirely sure if this is an issue with the VRaySun, the Clipper or ForestPro itself.