The higher the bit rate, the more processing power it takes for a computer to play it back. When it is too high for a particular computer, you get the results you are seeing. If it is choppy, you should try a lower bit rate. Also, try making your max bit rate a little higher than your target bit rate. This could help making areas of motion or change in your video look better despite a lower overall bit rate. Definitely experiment on short clips to see what your playback computer can handle.
Online video hosts generally re-encode your file to various bit rates and play back which ever one will play smoothly based on the users internet connection.