Hiya, I have a 2004 Golf MK5 / the car warms up fine and the temperature keeps idle but then the odd occasion the temperature drops, and the car revs go up, and then the after a little while the temperature goes back to 90 degrees.
when diagnosed " engine temperature sensor" error keeps appearing, I got the part ( one end is gold color and the other end is plastic in green, has a "O" plastic rubber around the gold side of the part, and also has a "U" clip with it.
It sounds like the wiring to the sensor is a faulty circuit. The wiring between the sensor and the Engine Control Unit, (ECU), requires inspection for continuity, shorts to ground, and correct input and output voltages. Once it is confirmed which of these circuits is faulty, it is a good idea to start looking where the main engine wiring loom leaves the engine and is attatched to the vehicle body, the area where the loom flexes when the engine 'moves' is more likely to be the place to find the faulty wiring/circuit.