The KM84 is not very sensitive to humidity, by itself. But being a KM84 it is certainly more than 24 years old.
Unscrew the capsule, take a soft brush and brush off any dirt on the white half-sphere of the amplifier, and on the rear side of the capsule. Make the breath-humidity-test before and after, to check for any success. If this doesn't help, contact our international service partners for a check-up with cleaning.
Best regards, Martin Schneider / Neumann Mic. Development