ADC0808 IC working correctly occasionally
Posted as an answer as there is too much for a comment.

There are a number of issues here:

1 The conversion start button has not been debounced, so there will (not may) be multiple (dozens) of start conversion pulses for each button press.

2 There does not appear to be any decoupling at all.

3 The LEDs need current limiting resistors.

Devices like this need a fair amount of support circuitry to operate properly. Given the schematic you have posted, I am actually quite surprised you get any valid results.

Getting different results from different ICs is hardly a surprise here, incidentally; subtle differences device to device in a situation like this almost guarantees that.







simulate this circuit &ndash; Schematic created using CircuitLab

Typical debounce circuit. Use a schmitt trigger buffer.