Depends on what you have in your tank... some things are more sensitive to heat fluctuations than others are.
I assume you've got your heater set at a certain temp? Are you having a hard time keeping the temp up, or does the tank get warmer than the heater is set at? If you're getting that much fluctuation over the course of a day, maybe you would want to consider 2 heaters to distribute the heating load. Just a thought.
With a relatively small tank (33g), I would bet that your problem is a faulty heater, especially if the room temp around the tank stay pretty constant. I trust the tank isn't in front of a window, a/c, or heat vent?
HTH