Kelly R wrote:
-------------------------------
what could be the possible reasons (and solutions)for this erratic behavior ?

NOISE !!!!

Don't expect ANY sense from it, with no input. Use a potentiometer from a nice clean supply (maybe even a battery) and THEN read the thing to validate what you have. You are only using an 8 bit converter. Look for a bad grounding scheme, separate the Analogue and digital grounds in the circuit, keep the processor quiet while you make a reading.....

READ all the copious notes here on ADC noise.

Steve