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My room is a comfortable 120VAC, but when I measured for DC, I got the following reading: It changed with each update between positive and negative 220mV or so, taking a few frames to switch all the way.

The reading were kind of like this:

220mV
130mV
40mV
-40mV
-130mV
-220mV
-130mV

etc. etc. Is this bad?

Nope.

What you're probably seeing is an instantaneous reading of a positive or negative excursion. Analog meters have too much mechanical inertia to display this phenomenon, but a digital meter with a fast clock will show up little tidbits like that.

There could be other causes, as well, but DC fluctuations in the millivolt range superimposed on 120 volts is really insignificant.

Later,

all right, thanks