Don't have one, but things to check:
Are your camera and thermometer set to the same emissivity settings?
Does the temperature change with angle you look at the surface (is it catching reflections)?
Do you have humidity and ambient temperature set in the camera?
Have you disabled the NUC shutter?
Is there a way to manually force a calibration shutter cycle?
Is the camera focused on the target?
If your answers are yes, no, yes, no, no, yes, then you're probably doing most of what you can do to keep it accurate. I would check the temperature with a thermocouple or other non-optical means to verify the reading, and then maybe complain to them if it's not meeting spec by this much (they say +-3C or 3% in the datasheet). Seeing the temperature trend downwards suggest to me that it could be related to the self heating of the sensor, which is normal, so does this exhibit the same behavior if it's been running for a bit (10-15 mins) and has temperature stabilized?