Ok, lets see...

1- take the snake to the vet to rule out RI

2- The largest tank even a big adult female should live in is a 40gal breeder. Most adults live well in 30gal breeder tanks. Long and low is the best. I suggest selling that 50 and getting a 30 breeder. Craigslist is a great source for buying and selling tanks. If you just can't manage to get a new tank now, then follow the remaining suggestions keeping the added size in mind.

3- Insulate the back and both sides with foam board, or at least cork board. As was said, glass loses heat fast and the insulation is necessary.

4- By thermostat, I'm guessing you mean thermoMETER. The thing that reads the temps. Because you say it's above the hide, I also guess it's not a digital with probe, which is really what is needed for snakes. Make sure you have a digital thermometer with a probe and set the probe UNDER the substrate on the glass right above the heat pad.

5- In some places, a second UTH (and second thermostat to control it) is needed for the cool side. You might need this to get the cool temps up to where they should be, especially in a larger tank.

6- The foil treatment should cover ALL of the screen top EXCEPT a square on one side for ventilation. The more screen that is covered, the more humidity stays inside the tank. Also, lights suck tanks dry so getting rid of the lights would really help.

That's all I can think of for now. See what you can do to get a new tank or fix up your current one and get back to us.