I assume you are using the Tobii Analysis SDK since you ask about the Tobii Pro X60 eye tracker and also about pupil diameter which is only available in the Analysis SDK. To give a relevant answer to your question I need to know a bit more about the circumstances. Could you please provide more information about:
1) Is Tobii X60 the exact eye tracker model name (I ask since we have other products with similar names, e.g. the X2-60 eye tracker)?
2) How much does the pupil diameter fluctuate and in what way (e.g. slowly drifting or random noise)?
3) Do you experience the fluctuations also when moving your gaze around a screen with uniform brightness? The pupil diameter is affected by the brightness so looking at a bright spot will cause the pupil diameter to decrease.