Do you have a drawing on how you are hooking this up?
Where is the power coming from for the potentiometer? Are you using regulated 3v3?
The core used to have a filtered power output that could help you here, but the photon did away with it. It needed the pin.
The weird thing is that I am doing ADC in photon without that much variation as you are seeing even with the radio in auto mode.
I fixed an issue rescently with some servos by putting decoupled capacitors in the power input of the photon by suggestion of users here. That was another issue entirely, the servo was starving the core from its power under high loads. However this servos where high power hungry ones. I don’t think it is normal for the radio to produce variations that big on ADC readings. I would also consider high pass filter capacitors with lower F rating… However…
I would check your cabling first. It the cable near the antenna ? Are the cables peeled open? Do you have cold sodders?
Try unhooking peripherals. For instance the OLED display. Maybe a component is serving as a big antenna and sending interference thru your lines.
I am not super good at this so I might be suggesting something stupid here, but hey !! It only takes time and not too much effort to try and a lot of upside of we find your culprit!.
Good luck and keep us posted.