Aquarium temp

Question
I'm starting my first freshwater tank in a 30 gallon tank I received from a friend. Its a complete setup, air pump, under gravel filter system and heater.
For over 2 weeks now its been cycling with no fish but the temperature keeps spiking to nearly 80 degrees when I have the thermostat set to less than 71 degrees.
Is there something I'm doing wrong or does it just sound like a bad thermostat in the heater?
The temperature in the room is generally around 70 degrees as well if that makes any difference.
Thanks
-Forest

Answer
Forest,

 Get a new heater than one doesn't seam to be working right. Also, think hard about using the under gravel filter system. They also have to be cleaned weekly and you end up dumping toxic water from the filter into the water. The ones that hang over the back are better, need less work and are safer for your fish.