maya batch random dark frames

Curious problem, I am batch rendering out a 1440 frame sequence of a plane flying.

Using a couple vray mats and the vray sun and sky in maya 2012/vray2. Using motion blur, vray physical camera and brute force GI, nothing fancy.

Every 6-12 frames I have a frame that is darker than the others, it’s random. Almost looks like on those frames there is a different gamma. Any ideas?

I’m on osx lion, 8 core macro with 32GB ram I am rendering locally.

Can you upload a picture illustrating the difference?