Piezo Electric problem

Hi.
I am designing a circuit to get signal from piezoelectric, amplifies it and then convert to digital by ADC0804. The data collection and conversion is done at each 200 Micro second.
there is a problem in this circuit. the output data is same as follow:
Weak pressure on the piezo:

Data: 00-00-00-00-00-00-00-00-00-00-00-57-1F-30-21-75-01-11-00-00-00-00-00-00-00-00-00
-------------------------------------------------------------------------------------------------------------------------->Time Axis


Strong Pressure on the piezo:
Data: 00-00-00-00-00-00-00-00-3F-DA-00-80-DC-00-00-50-00-00-00-3F04-02-0F-00-00-00-00-00-00-00-00
----------------------------------------------------------------------------------------------------------------------------------------------------------->Time Axis

Problem:
Why the data is alternate between high and low?
at the first example the data is 57(High) and then go to 1F(Low) and then go to 30(High) and then 21(Low) then 70(Very High) and so on...
why the data alternate between low and High?(Why it is not incremental???)
I want to measure pressure on the piezo but this data is not valid pressure(I think).
(I use op amp 741 and adc0804 to amplification and conversion. I am connect piezo to invert(-) pin of op-amp 741)
I think the output of piezo is curve like (-X^2). is it true or not???
Please Help me.
thanks.

1. Sort out the analogue domain first. Does the output voltage of the 741 indicate correctly with respect to relative to the piezo pressure? Is there any noise on its output?, oscillations? or any other possible cause.
Once you are sure that you have a good voltage to measure then re-check to see what your ADC is doing, it is possible that the 741 is actually picking up radiation from the ADC and causing an analogue to digital to analogue feed back loop and oscillating.
If there is a problem at this stage try to feed the ADC from a variable voltage source (a pot across the supply volts?). Then with only the voltage supply and the ADC you can fault find the ADC circuit.
Frank

A 'scope picture of the raw input is worth a thousand speculations.