The two obvious things that spring to mind are...

(1) If you have a 20mV or so ripple on your DC/DC converter, and this is your +5v Vdd, and in turn this is also your +VRef for the ADC's... your ADC's will immediately jump about by the very fact the VRef isn't steady.

(2) You didn't say if you had a filter in-line with your ADC inputs. You can't just take a Sensor voltage coming in off-board from some distant location and plumb straight into a raw ADC pin. A simple RC circuit usually suffices.

Melanie