frozen pattern within fire

[finalight]

I'm getting a frozen noise pattern down inside the fire of an explosion sim. I haven't seen this before version 4.0 and am trying to isolate it. I've tried many different combinations of render steps, jitter, Vray AA, etc. and am not able to remove it. This sim has a lot of detail, hence it's not due to step/voxel issues from a low resolution sim. I'm using version 4.0 on this box. I'm going to test further, however any ideas would be helpful. Here's a screenshot. Thanks!