Bottom 20-percent D70s image dark

I have a D70s and recently encountered a problem whereby the bottom
20% of the frame of some shots were dark. What I did was I set the
camera on a tripod, set the self-timer and got the shot. I saw in the
LCD panel the darkened bottom and, leaving all settings the same, I
set the self-timer again and got another shot and then the pic came
out OK. Then the next shot was fine and then I saw the dark band at
the bottom again on the shot after that. No fingers and no lenses and
no filters and no hoods, etc. were in the way. And no flash was
used.

The camera had been sitting in a house (on a table instead of in a
closed camera bag) all day where the humidity reached 60%. I don't
know if that could have been a culprit or what.

Any insight on this?

Thanks.
Kevin