Contrast Problem in XBMC / SPMC - Dark / Brigth changes (floating blacks??) topic

Hello Guys...
my problem is mentioned above!
I got this problem only when using FireTv not when exchanging it for my htpc.
Hardware is
AV Receiver Yamaha RX V2067
and
Samsung D6900 Plasma.
What can be the reason ? I got it often on scene changes in Movies and it is very annoying...
Is it possible that the cable is the problem?! Although it isn´´t a problem on HTPC so far.