If the material you're trying to watch frequently buffers, it means that you're not maintaining a fast enough connection to our servers to watch our content at the speed you selected.
NFL Game Pass requires a minimum connection speed of 300kbps to watch video. On the player, in the upper left-hand section is the "Preferences" link. Clicking it will present you with the different bandwidth options available. Selecting a lower speed than the currently displayed speed will restart the video at a lower bandwidth setting. This should reduce or eliminate the buffering.
If that doesn't solve the problem, you should configure your player to accept HTTP streams. Here's how: