You can try raising the top bar closer to the top of the doorframe.  Then, you can also raise the bottom bar accordingly so that the punch pad is at an appropriate height for you when it slides down. Increasing the distance between the bars will increase the tension of the bungee cords. 

As far as finding the correct rhythm of punches, this does take some time and practice.  

Please take a look at one of Brian’s Live workout below, to see his set up and follow along with his rhythm.