Try extending your exhalation, for example 4 - 1 - 6. This will allow a more controlled pattern of breathing and better relaxation. Eventually you will learn to inhale and exhale in a manner, and at the right time, that will minimize the "up and down effect", somewhat like steering a single engine boat with a serious yaw, beginning to exhale just as the upward movement begins. I don't think the timing part can be "learned" quickly but rather is something that comes with experience and without conscious thought. Also be certain that you are swimming in a horizontal fashion (level trim). If your head is high, each time you kick part of your effort is in an upward direction which will magnify the up and down movements.