It shouldn't flucuate, at least mine doesn't. Maybe I am lucky with mine. The most it ever flucuates is a degree or two but thats it.

What are you using to hold the probe in place? I have learned the hard way to keep the probe on the outside of the tub/tank.

Houdini has moved his and the temps went haywire. If the probe is off the heating device at all it will flucuate quite a bit.