Me again.
Bought a small 9 V power supply, connected it to the sensors. The logomatic gets its voltage through a 5V regulator. Now the measurments are more stable, but still 35mV / 3.5 centigrade off. Seems the 3.3V voltage is a bit whacky. Can I supply the 3.3V externally? Shall I crank it up to 6V?