Sometimes while driving there is a vibration at forty to fifty MPH

When driving (not all the time), sometimes I get a "bumpy" vibration at approximately forty to fifty MPH which ceases when speed is varied and may not occur again when resuming or slowing back to the forty to fifty MPH range. At first I thought it might be a wheel balancing issue, but than thought of a possible drive line universal problem. Can you please help? Many thanks for your response.
Freeland, MD