Temp fluctuating

electric guage uses a known value variable resistor sensor 10-180 ohm or some such
the resistance of It and the wire from it to the gauge is impacted by

1) how well its threads cut through the sealnt into the head or housing
2) how well the wire connects to it
3) all of the junctions through the bulkhead connector to the gauge

a loose wire to the sender moves as the air flow through the radiator increases
if this movement causes the resistance seen by the gauge to chanage the temp recorded changes.

rsistance at any junction will cause a higher or lower reading as well.

the two things togther will see what is shown on the gauge become dependent on speed humidity and ambient temperature alongside the actual temperature of the coolant

clean and re clamp all connections and it may clear the issue