The problem is you are much too close to the minimum the sensor AND the adc can measure ...
moreover you are close to 10 ADC counts ( IF 10 bits used ... ) and the jitter is ... +/- 1 LSB !!!

no need to talk about 8 bits conversion ... ???

so, guaranteed 10% error ... @ minimum !

try a LM 335 instead ( voltage will be 2.79 v for 6.0°C ... ) and the Pic ADC will work perfectly ...

Alain