There are some heavy loads involved. The circuitry is a thermostat, that sets heating power of a resistor by cutting power on and off via a N-fet. The fet, however, is connected directly to 12V input, while the PIC is powered with a 78L05 regulator. I guess I need to scope the power to the PIC to be sure.

I tried to chech the output from the INA with a data logger. Did not see any ripple there, however the logger had acquisition time of 1ms, so I suppose I'm better off scoping that part, too.

Software measurement rejection is indeed one option, I just wanted to make sure that there is no error in the A/D configuration before taking that route. Thanks for your comments!