My temperature gauge on my 2lt diesal is misbehaving!! I noticed it not recording normal temperature staying in the cold section,I determined that it was the thermostat! My garage replaced it while having a service, when I drove it away it was showing normal temperature, however after a few miles it started dropping? I was driving into London and in the usual traffic it held normal temperature, but on the journey home when on open roads and motorway the temperature gauge went to cold? Mystified is the word! As it help temperature in traffic one could assume the stat is working so why does it drop when on motorways. now it’s stays at a quarter of the gauge?