Strange Frequency in Sensor Data
A bit of speculation:

If you look at figure 5 in the data sheet for the TMP36 you can see that load regulation is broadly 0.006 degrees per micro amp drawn: -



If during the ADC process the current peak is (say) 1 mA (certainly not unheard of) then you will get an error of 6 degrees.

So, if your ADC is a multiplexed and unbuffered type that draws a spike of current during conversion then you'll get odd results. If you applied the maximum capacitance of 10 nF across the sensors output to ground you may well see this error fall dramatically.