Hallo Hans,

you cannot test such a noisy system with an input voltage derived from a potentiometer, perhaps with long, non shielded wires hanging arround. There is so much EMI, that only touching potentiometer with fingers will cause very strange performance. You need a circuitry consisting of massive groundplane, with adequate routing of analog and digital ground, adequate power supply decoupling, adequate low pass filtering at inputs, adequate shielding, and driving inputs by adequate low ohmic impedance. May be that just a 10nF/X7R capacitor at input of ADC will help.

Don't trust this simple and stupid datasheet, which tells you that you can directly connect some Wheatstone bridge without any additional measures like filtering and shielding. It's always necessary to 'play the full program' when dealing with ADCs.

Good luck,
Kai