Temperature indicator fails while driving

Hello, I have just looked through some forum entries, but I can’t find a solution for my problem. Therefore this new entry. My problem: When I start the car it will get quite normal warm during the ride. The temperature indicator is as always horizontal. After about 10-30 minutes the temperature indicator falls to zero at once and does not go up. Before about 1000 km the thermostate housing, thermostate and the temperature sensor at the thermostate was renewed and up to now also had no problems. (supplements were from Skandix) If I lengthen the cooling hoses, these are as always. Via the cooling temperature sensor the fan (behind the cooler) is regulated, right? What can be defective, is there any relay? About your help I would be very grateful. Greeting Georg