Also LN2 temp is not that stable, it vary about 0.1K over minutes. Dewar is not pressured, so likely LN2 getting liquid air and oxygen causing temperature to vary.
Liquid N2 normally is quite stable, indeed.
In fact, pure liq-N2 (~77.2K) condenses air, and therefore, the oxygen part of this mixture will slowly raise the temperature, towards the 90K of liquid air. But that takes hours to reach maybe 80K, depending on the volume of the dewar, and the temperature reading will not flicker on minutes basis, instead goes steadily upwards. Anyhow, you should cover the opening of the dewar with cork, let a small hole anyhow, for the evaporating gas.
Also, changes of atmospheric pressure may slightly change the temperature of the liquid N2, but that also does not happen within minutes.
Your sensor is not of best quality, and very sensitive. A PT100 will deliver much more stable readings.
Frank