What brand is it?? I got mine at PetSmart (whatever their brand is) for 20$. It's a 25watt adjustable. It's in the kitchen window which has a slight draft and is able to keep my tank at a steady temp.
What is the wattage of the heater? For a 10g you might want to go above the suggested heater wattage (is it 50W per 10g?) so it doesn't have to work as hard.
It could be there's some malfunctioning hardware in the heater. I would return it and try another. If you have the same problem perhaps go up in wattage.