I had similar issues and it's related to a number of factors, the main one being that your Chromecast is not receiving adequate wifi signal for the quality of the stream in your tv. As you get further away from the router the wifi attenuates which could be the cause of your buffering. Wifi also is subject to interference from objects, neighbours wifi etc and will be less effective if there are too many devices connected to it.
Some options
1. Check your wifi speed with the internet connection test built in to the Google tv You need at least 5Mbps for hd quality but usually much higher the more stable
2. Try to place your modem closer to the tv if you can and away from anything obstructing like walls or furniture.
3. If that doesn't work, can get a wifi repeater/extender. My experience these don't work that great but I had a cheaper model
4. You can make the Chromecast wired -
for a cost you can get an adaptor with ethernet from Google or third party. Then You can either extend a ethernet cable from your router to the Google tv which may not be feasible or you can set up a wifi bridge. A wifi bridge is typically another router or device that you keep close to the Google tv and connect via ethernet. The bridge then connects wirelessly to your modem and provides a more stable connection to your Google tv. This is what I've set up yesterday- seems better so far butlet's see how I go.
PS: I am a volunteer community member and not affiliated with technical support or Google in any way.
