I had a similar problem with my render which I posted on
Someone had a good idea, maybe this will help
"Try upping the Final Gather rays to 500-1000 maybe. If that doesn't work , increase it to 2000. But it might be that the actual model is too small, or too large. Since MR uses actual Model size and size characteristics for FG and Photon emission calculations, if your model is longer then 20 units or smaller then 3 or 4, then you should scale it to be between those two values."