[Elecraft] Zerobeat issue

[rocky cookus]

Was on 40 meters trying but unable to zerobeat a CW carrier with my K3, and
noticed that the gradations in tone from this signal were not continuous
but were in intervals.  QSY'd to 20 and the signals were OK, and when
coming back to 40, was able to spot tune a CW signal.
This is an intermittent issue and not a common one, but it does happen from
time to time.  Any ideas would be appreciated.  Perhaps there is some sort
of intermod mixing that creates this distortion.  Anyone else notice this
problem?  If I had to graph this problem, it would be with a step function.

If it were a musical instrument producing these non-continuous tones, I
would say the instrument is unable to play chromatic scales.  This issue
typically goes away quickly, but is puzzling.