If your movie stops in the middle, it's likely because your device's cache or network is too busy. Many stalls can be fixed by refreshing the app or page, updating your browser, and clearing your cache.
Make sure that where you watch, your link is fast and stable. To avoid buffering, try your speed and aim for 5 Mbps or more for HD and 25 Mbps or more for 4K.
If viewing stutters in a browser, turn off and on hardware acceleration in Chrome and then start it up again. This fixes a lot of video problems on all devices.
DNS that isn't reliable can make stream starts take longer. Try a reliable server like Google Public DNS and test again. Change back if performance doesn't get better.
Unofficial or sites with a lot of ads tend to freeze. Licensed services, on the other hand, use better CDNs and are more stable around the world. If you can, switch to a legal site to watch without any problems.