Fluctuating engine coolant temp gauge

I have a 1995 XJ 4.0 petrol. There are 5 lines on the engine temperature gauge. Line 3 is the midway mark where the temp gauge typically sits. I drove a round trip of 360 miles in a day recently. On the way back home on the highway I was glancing at the gauges and noticed the engine temp gauge was visibly increasing to just over the half way mark on the gauge, and then immediately dropping back to a point midway between the 2nd and 3rd lines on the gauge. It would then repeat this visible heating and cooling cycle over say a 1 mile stretch. It did it the entire way home, and since then I have noticed it happening reguarly on any trip I do.

When I stopped moving, the gauge seemed to remain steady, so I'm wondering what might cause this?

It goes without saying that you checked your coolant level, so likely the thermostat is sticking. Cheap and easy to replace. With better airflow at speed, when the tstat opens, it lets in coolant at less temperature than when idleing.

on your dial i suspect 100 translates as 210. And on the oil pressure 40 is the midpoint.

I'm in Australia, so yes my gauges are metric readings.