Well, here's a test for anyone with a Galaxy Nexus (especially with that problem) that still owns his older Android phone:
Put your SIM card back into the old phone, configure it to GSM only, put the Nexus in Airplane mode (to disable everything, including emergency mode (I think)) then put the old phone with its ass close or next to the volume rocker of the Nexus. Maybe even overlap the phones, to have the antenna on top of it. GSM antennas are required to be at the bottom of the phone. Fiddle with the old phone to generate some radio traffic.
If you can manage to cause the volume drop that way, it's definitely RF.
--edit: I guess given that the wavelength of 900MHz is around 33cm, you don't need to be too exact with the placement, but right on top hammers the most power into it.