MR FG weird flickering

[Judicator]

Hi, guys.

So, I've got a little problem I don't know how to fix. Maybe some of you have ran into this one before.
Basically I have a material with transparent shader, connected to "Surface" port, and a color mixer node, connected to "Volume" port.
When I render everything out, sometimes (on some frames) weird flickering appears, especially if that object has very uneven surface (right cube in the picture). Usually it's irregular shape white spots. And that happens only when Final Gather is enabled in mental ray settings.
Any ideas on how to fix it?
I'm adding my material setup as-well.