What can I do to avoid shadows flicker?

Hello guys!

I experience a problem with shadows in modulative mode. They are flickering and it looks like the car would be drawn two times, but I checked everything to eliminate any possibility of that.

Have you any ideas what causes this?

Another issue is why shadows are rendered like volumes through the air? I don't mean the greenish shadow volumes you can let render. It's possibly a deeper render engine problem, but also any ideas or comments to that?