Like I said in my earlier post to you, insulate the tank with foam board, and place foil over the screen lid.
Use one light on the cool side to bump the ambient and keep the UTH. You might have to increase the watt of the bulb, but for now, try what you are using. It should work.
Back when I kept one Ball Python (a very long time ago), I had him in a 40 breeder. My room would drop to 66 degrees sometimes. With the insulation and foil, his tank never dropped below 84 ambient/93 basking.