What is the wattage on it? You would at least need a 300w heater to work for a 100g tank. 75-82 is definitely fluctuating too much. I run MH
and my temp goes from 79-81 consistently. If you are running your central heater, make sure it is not blowing directly on the tank, or in direct sun light.
Also, set your heater to a temp that is least likely to fluctuate. I keep mine at 79, so there is only a 2 degree change.
You could get 2 heaters (150wx2) and set them both to the same temp. Yours may end up having a problem, and having the second one will help minimize problems if it ends up sticking. HTH