Rajiv,
For consistent and very accurate results with ADC's you need a very carefull PCB layout and need to kill the noise at every possible occassion read the noise eliminitaing techniques in the hardware faq section.
http://www.8052.com/faq.phtml?FAQ=84

Also if its possible can you please post the exact schematic of you circuit. & why are you using a 555 for clock can't you do it with your micro. What about your refernce please specify in details

Prahlad Purohit