How to stop apple tv from buffering

The reason Apple Tv buffers is because it running out of video to continue playing. i.e the video isn't downloading fast enough to keep up with playback. So it needs to stop playing, and buffer more video before it can continue.

This is usually due to a slower internet connection, a very unstable one, or much less often a problem at the server where the video isn't being output fast enough.

In either case there's not much you can do on the Apple Tv itself.

Even though you mention your connection is fast enough(18mbps should plenty) i would try some tests. Try playing videos from places like youtube, that you know do not have and HD version. Something around 480p or such that the apple tv cannot try to stream the HD quality version, and see if it buffers. If it doesn't then that's a good indication the internet connection is not keeping up with the higher quality videos and you would need to address that with your ISP.

It might also be a router or base station problem, especially if you are running an old one. Try unplugging it and let it sit for a minute, then plug it back in. I had to do this on my old Airport Extreme a few times when I had WiFi issues.