Salemn:
I would also look at the positioning of the heater, the positioning of the aquarium, and the hood/cover/light kit. The heater should be placed next to or in the vicinity of the filter water flow or other circulation...like an airstone. If the aquarium is next to an outside wall, heater, airconditioner, etc...this could affect the temperature stability. A good fitting hood helps alot too...not just with temperature stability, but with drafts, evaporation, etc. You may also consider getting two smaller heaters to do the job...to even the heat by placing one on each side of the tank. Then again, it could be the size of the tank. Generally speaking, the smaller the tank, the less stable the environment.
Take a look at all the suggestions posted...and I am sure you will solve the riddle.