loose nut behind the steering wheel

Move towards cooler side, how? It just marginally goes cooler or does
it peg to the cooler side? Does it immediately move or take a couple of
seconds.

If it just marginally and slowly moves (ie within a couple of second),
it is just a function of more air moving along the engine/radiator and
thus cooling the engine more.
It may dip a little, but the car should stay within certain temperature
margins to run correctly. If it dips a lot and stays there, it could be
that your thermostat is stuck open and the engine is just running too
cold. Does your car take longer to heat up lately? If so, that is also
due to a stuck open thermostat.
This is not a catastrophic failure, but I'd replace it because your
engine will not run correctly. A stuck closed thermostat, on the other
hand, will cause overheating.

If your temperature gauge just dips immediately after you start
driving, your temperature sensing mechanism might be faulty - most
likely an electrical problem. Immediate dips (ie. zero time) are near
impossible to be caused by temperature.

Hope this is useful.
Remco

Look at the wire going to the temp sensor.  Mine was loose where it
connects to the sensor on the thermostat housing.  I just gave it a twist
until it was tight again.  It is riveted to the end of the sensor.  You'll
understand when you look at it.