Mental ray overlapping grids

[dr_ron]

Hi
i am having issues with overlapping Fume grids with mental ray.
We are currently re-doing a project from final render to mental ray.
Final render with Fusionworks is fine.
Now with mental ray im getting black box with harsh line in the overlap.
This is just a small test scene to show you the problem.
Default MR setting in Max with Autovolume on.

Running:
Max 2014
Fume 3.5.4

Any suggestions?
image uploaded.
[ATTACHMENT NOT FOUND]

thanks for any help

ps...we have to use mental ray :$