Maybe loose splice on neutral wire? But this would also
cause some lamps to sometimes glow brighter and some
incandescent bulbs to fail faster. Defective neutral is also
just another human safety reason why the earthing ground rod
is important attached to breaker box.

Symptoms are getting squishy. Once it appears contained, it
squeezes out somewhere else.

Choreboy wrote:
Oh no, 240! The affected outlets represent both sides of the
transformer! There goes my speculation of a loose splice on a secondary
line. I don't know why so much in two houses had to be reset but
everything at my house was fine.

(It's been years since I looked inside a breaker box. That's why at
first it seemed natural to me that all the switches on one side would
come from the same side of the 240-V supply.)