Ok first off I would just unplug the heat pad unless you get a thermostat for it. It can and will burn your snake. Second I think your thermometer is the problem here. Most of those types of thermometers are inaccurate and usually measure the temperature of the glass and not the air. I would go to Wall*Mart and buy an indoor/outdoor digital thermometer with probe. They cost $12 and work much better. A 100watt lamp is more than enough to heat a 15 gallon tank so I think that something else isn't working right.