Your main problem is probably coming from the fact that you have a 55 gallon tank. These tanks are made for fish, not reptiles. You will find that your 55g tank is very tall. Your ball python is a ground snake, and will never utilize more than 12" in height. That means that you have to heat all that extra air that never gets used.

Besides the strange dimensions of a 55g, the glass tank combined with a screen top will not hold heat or humidity in very well. You are better off with a specially made reptile enclosure or a simple plastic tub. I know a lot of people like to see their animals and put them on display. If that is the case for you, look into a reptile enclosure. You will have a much easier time regulating your temperatures in this type of enclosure.

Also, what kind of thermometers are you using? The type of thermometer you are using should be digital with a wired probe. Any other kind of thermometer is far less accurate. So while your stick on gauge thermometer may read 73*, it could be as high as 83* in the same spot.

If you post a picture of your setup, we could give you a better idea of what you can do to fix your temp issues.