Temp gauge fluctuation

Temp gauge goes up and down being slightly above the
middle line to below the middle line. Tends to stay below as
long as I am driving however does so at times when ideal
and that is when the heat does not pump out at its fullest
just barely warm. The heat also is up and down hot in
which one feel comfortable to warm with a slight cool
touch. The thermostat has been replaced and the radiator
coolant tank.was kind of rusty but fluid replaced and not
flushed. Also PCM programme reprogrammed by
dealership and full diagnostic before hand and the code
came up to reprogram PCM.