I have a Panasonic Viera Smart TV and after upgrading to BT Infinity and getting i hugely faster connection i thought Netflix would be OK. I was wrong. The sound would start to fade and then then everything went out of sync. I had my streaming setting on maximum as the connection would easily support this but that was the problem. Log into your netflix account and at the top right go to your account. Look down at playback settings and drop it from High to Medium. It worked for me, hope it does for you.

How is the TV connected to the router? If you are using WiFi then that could be the problem not the Infinity connection. A direct ethernet cable connection should be good enough though.

Mind you there could also be a load of external factors outside of your control between Netflix and your telly that could account for the problem.

I've had zero issues with panny tv through wifi and ~8Gb download speed.