Are you measuring the temperature of the tank on the side or in the air, or are you measuring it on the bottom of the tank? If you have one of those stick-on thermometers then you're measuring the temp on the side. It's the bottom of the tank that needs to be about 90-92. Frequently the heat pads get too hot, actually. You will do best to get a digital thermometer with a probe that you can put on the bottom of the cage on the hot side. That is more important than the air temperature as long as you heat your house "normally" in the winter (say 62-68 degrees).
Substrate --there are many choices of non-particle substrate including repti-carpet, ceramic tile, newspaper, paper towel. Many people (me included) feel that sand and other particles are dangerous substrates for leopard geckos but there are others who disagree. In my opinion, better safe than sorry.
ALiza