water temperature gauge

Some times my water temperature will just drop off.
I have a obd1 reader installed and when I check the temperature is reading on that so it's not the sensor and not a fault before the ecu.
It must be a fault somewhere between the ecu and gauge.

My question is.

Has anyone else had this and what was it?

Could it be the actual gauge that's faulty.

The needle will drop and then raise again..
Some days it's fine. Other days the needle stays down.

Gauge is separate from ecu so you need check whole circuit including the gauge single wire sender. could be as simple as sender wire connector, sender or gauge intermittent open .
Regulated voltage to sender is either 8V or 9V not sure which without testing one. as sensor heat resistance drops and more current flow to earth drives needle up (sensor earthed via block) .

Thanks B.
I will have a look at it monday.