Problem: House pressure rises during the night from 40 PSI to 70 PSI.
Is incoming pressure regulator failing?

Water meter pit is 175 feet from house. Pressure regulator is in
ground next to meter pit. City water pressure in meter put
approximately 100 PSI (according to city). Regulator is adjusted so
that incoming house water pressure is about 40 PSI as measured on
gauge in house. Pressure gauge in house is located on the house
piping as it enters basement from the outside. Downstream of this
gauge is check valve followed by thermal expansion tank. Also have
pressure gauge screwed into hot water tank drain valve. Note that
pressure readings on both gauges are approximately the same (within
the accuracies of the gauges).

During the day when we are home and using the water, pressure is
relatively constant from 38-42 PSI (within the accuracy of the gauge).
However in the morning before we use the water, the pressure has
risen overnight to 65-70 PSI.

Prior to leaving on a recent vacation, I shut valves in basement and
released pressure on house piping. The pressure on the incoming water
piping pressure gauge was 40 PSI and the pressure on the hot water
tank gauge was about 0 PSI. When I returned from vacation, the
pressure on the incoming water piping pressure gauge had risen to 70
PSI and the pressure on the hot water tank gauge was still at about 0
PSI.

Does this indicate a problem with the pressure regulator? If so, what
is going wrong with the regulator mechanism? Should I replace the
regulator?