When settings up a modem for IPTV and you create an Interface Group there is one additional step to keep in mind.
After the interface group is created, go to Advanced Setup\LAN.
From the drop down menu at the top right, choose your new IG – usually called IPTV.
Make sure to set IGMP Snooping mode to Standard Mode.
If it is left in blocking mode it may cause the STB to get stuck at 11/20 or intermittent pixellation.