Where is the inaccuracy of my arduino thermistor circuit coming from?
Why are you using the voltage divider and measuring by parts? As @nickgammon points out, you are measuring only a 1/5 of full scale at each measurement, meaning your resolution is down by 5:1 over what the A/D is capable of. Also, the divider network comes to around 500KOhms. Work out the current through that and you get a very feeble &lt;7 microAmps. The wires are antennas that could well be contributing noise at a significant fraction of that level.

Is there a reason for the divider circuit? Could you use a single thermistor from Vcc to Gnd plus a resistor in a 2-way divider? That will have the advantages of:


Using (more of) the full range of A/D;
Taking only one reading instead of several (and the several are necessarily skewed in time since you're multiplexing the one A/D);
A larger current (ok, only 33 microAmps, but 5x better than before);
Presumably less pickup into the network than with 5 leads. Keeping the measurement lead as short as possible may help, too. 


Update:

Then you probably want a 2-component divider for each temperature you need to measure, paralleled between the power rails. I just guessed, not calculated, the resistances of R1, R3, R5, &amp; R7. You'll want to estimate the maximum and minimum temperatures of interest in each channel and pick values that will give you the best ranges without drawing excessive currents when the thermistor values are low.







simulate this circuit &ndash; Schematic created using CircuitLab