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 

I had the same problem with mine (2008 2.2d 6sp manual), I ended up replacing the oil coolant thermostat and everything worked fine. 

There is a well read topic on this forum on the very same scenario. 

HTH

Thanks for the reply. You are correct I changed mine and the problem is solved. Also changed the main thermostat and head temperature sensor and water pump while I was at it.