Chopper stabilised op-amps and noise
The noise of the amplifier you are using is typically several uV p-p from 0-10Hz according to the datasheet. 

Chopper op-amps also use switches right at the front end connections so there are huge (relatively speaking, compared to the average) transient currents present at the input connections. If your source impedance is large, that can affect the signal. Note the specsmanship present in the datasheet: 



Assuming you have a non-inverting amplifier configuration and don't need a lot of bandwidth you can try a large-ish low leakage capacitor from the non-inverting input to ground and across the feedback resistor. 

I assume you've bypassed the supply properly as you refer to it, but double check. Adding capacitance to the output will probably not do anything that is helpful- you are decreasing the stability so any noise will cause ringing. 

Edit: Keep in mind also that chopper op-amps tend to have high sensitivity to EMI- since you have a switching supply this could contribute to the noise. There are some newer models of chopper amplifier that feature EMI filtering on the inputs.