I have a 2003 Chevy Impala. With 120000 km (75000 miles) on it.

Up to a year ago, the temperature gauge would always stay in the
middle of the range. Recently, the gauge stays about 10 degrees (20F)
lower than it should be.

Engine check light came up. The mechanic checked and said it was
complaining about thermostat and oxygen sensor. He reset it and things
went fine for a couple months or so.

The temp gauge only gets to the middle when the car is stopped.
Outside temperature has no effect, and the heater and the A/C work
fine to me. Perhaps the heater is not as hot as it used to be, but I
am not sure.

The light came up again a week ago. This time I haven't taken the car
to the mechanic yet.

I checked the coolant reservoir when engine running and there are a
few small bubbles coming up. One every second or so.

There are 3 possibilities that I found on the web:
1. Head gasket failure. This is unlikely coz the car runs fine and
there is no water in the oil and there is no oil in the coolant and
there is no overheating, even on long road trips. I have no coolant
loss either.

2. Radiator Cap failure. This could explain the bubbles, but does it
lead to running colder too?

3. Thermostat stuck open, or half open. Is is possible at all?

Please let me know what other causes may be there, and also if I am
correct with my observations. Thanks.