Actually, one could substitute any crystal from 8 MHz
to probably 20 MHz. Theoretically, running the arduino
changes the voltages to about 3.3v, but I and others have
noticed it works fine on 8 MHz and 5v. I would avoid anything
around 12 MHz as that is the USB frequency range and might
interfere. Easily had is a 10 MHz crystal which would probably work
fine. Even a HC49S type would work with just one jumper wire for
one lead (or maybe soldered direct), and the other pin soldered directly.
The old crystal would. of course, need to be removed.
A simple solution is to do what Jerry suggests, and change the BFO
frequency slightly. That may or may not cure the "symptoms"; it certainly
will not cure the problem.
It is likely that uPC crystal radiation is the problem, however. But it may be coming
from inside the radiuno itself. Sometimes (often) this because of Rf leakage back
through the power supply; make sure that is well filtered. Farhan's suggestion
of crystal aging is also very apropos.
Probably the next simplest solution is to replace the nano...