What could cause the temperature gauge to fluctuate?

I put key in ignition this morning, temp gauge fluctuated went right into the red then back down and back to red about 5 times. Started engine continued to bounce around even while driving

I took coolant cap off but it still fluctuated. It did this for a while last year but stopped for a while but then I noticed it fluctuating again today.

car is a ford focus 2006 mk2 sport 1.8 tdci

any ideas to the cause?

thanks

Added (1). Is it possible a crack in the head or engine block could be the cause?

Added (2). It happens after I get in my car after it has been parked a while such as over night and put the key in the ignition, before I even start the engine it fluctuates, and often continues to fluctuate after starting the engine

God created ****, Ford put it on wheels, of course you are going to have these problems

That seems like a loose wire to the sensor. All this time, you never had it looked at indicates how you maintain your car- or, rather don't maintain it.

Unplug the coolant temperature sending unit and see if the gauge still does that. If it does it's likely a bad gauge or a short in the wiring harness.

Replace the ECT sensor.

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