writes:
[snip]
As has alrady been suggested it may be interference, since it works
close to the NDB.

You say ' the needle tracks the NDB if I am near' so have you listened
to the audio out of the ADF receiver? Maybe you can identify some
interference.

Sounds like a similar problem, we had, which was due to alternator
noise (a whining noise which varies with engine RPM). In our case an
alternator decoupling capacitor had not been fitted with the
replacement alternator which did not have an inbuilt suppressor.

Onboard interference source sounds very likely. The ADF
may even be giving a clue as to the general direction of
the interference source from the antennas and may help
isolate it. [Note the "may" in the statement.]