The problem with the WI-FI speed

Connect to Wi Fi using the phone. For test download any game and watching the download speed, it ranges: 800-900 KB/s, but does not fall below 200 KB/s. Run a video on u tube in 720p, watching my speed, when you start the video the speed 800-900 KB/with and videos loaded smoothly after a few seconds drops to 50-100 KB/s, when stopping videos (because it does not prigrezilos therefore stopped) the speed increases again.
So it is impossible to watch videos or listen to music, speed is there, but it is not at all stable. Help solve the problem.