Analogread inconsistency on noisy power supply

I’m getting wildly fluctuating readings on an analogread when on the same power supply an Arduino Uno reads consistently.

The same power circuit on an Arduino Uno delivers precise analog reads, only varying by 1 (equivalent of 4 on Photon) while on the Photon it varies by up to 150 (/4095). When I change the power to the Photon to a 5V battery power supply I get cleaner reads (varying only by 10-15 (/4095).

The Photon is obviously much more precise at reading than the Arduino (but the precision causes headaches). Hooking it up to oscilloscope I can see that the A1 voltage reads from 2.40V to 2.51V. Now the Arduino seems to be great at flat lining that to one value (2.42V) even when I only do a single read. But the Photon seems to read it ‘too’ precisely capturing different points in the voltage variation.

It takes a lot of oversampling to approach the consistency of results I can get from Arduino. Is there a bit change setting for the ADC that would help or would that effectively end up being the same as oversampling?