Programmable S2C : ADC drifts with temperature?

Using the programmable xbee with an NTC thermistor attached to input AD2. Measures correctly when electronics is at room temperature and varying NTC temp in oil bath.
Tested with a fixed 3k48/15k resistor and used cold spray on the xbee. Temperature drifts by about 1°C. This is not acceptable.
Applying 2V5 on VREF Pin 14. But the S2C manual states this pin is not connected ??

Question: which processor is measuring pin 18, and which Vref does it use?
Is it expected to be so varying with cold temperatures?
What approach to take for more stable Vadc readings over -40…+85°C ?

Your inputs are appreciated, thanks.