Max31855k - fluctuation and drift
Turns out it was indeed the lack of the 10nF capacitor across the T+ and T- pins.
With the capacitor added, the temperature measurement is &quot;rock solid&quot;.
Reading the datasheet, I mixed up the 0.1uF and 10nF capacitors and assumed (yeah, mother of all f...) it is the same thing.
Hopefully others do not make this mistake :)