always the last digit jumps around
of course it does!!!
take as an example an 8 bit ADC where 1V raeds 047h and 1,01v reads 048h. what do you expect if the input is 1.005V/ a tolerance/stability of a picovolt???

there is a fallacy that ADCs circuits are as good as the ADC, and the porecision is as good as the resolution, when I hear that is always ask "get me a muckraker and a pair of boots".

have you read http://www.8052.com/faqs.phtml?FAQ=128848 I guarantee you that even if the last bit stopped "jumping around" your precision would be nothing like you think.

scales that have an actual precision (do not read 'resolution' DO read precision) cost in the order of $10k, where did you get the illusion that you can do better for a few bucks?

Erik