Temperature Gauge up and down / VW Gol G3 1.0

Dears all,

Recently, I’m facing strange behavior in the temperature gauge because it flaps up and down quickly between 0 and 90 degree (Celsius).

Could you please advise about the reason of this problem, taking with your consideration the following points:

* The engine does not overheating.

* The radiator fan still works

* The heater (air conditioner) is not warm enough.

Car Model:

VW Gol G3 2003 1.0

Thank you