Yea my 2.0 is doing this and waiting to get a new thermostat fitted, just under the air feed pipe in the right hand side of the engine, its on the side, has three of four pipes going into it, the temp sensor is on top of it.
Mine will sit at 90 in trafic, but then will go down to 70 even 60!
If i dont let the car warm up first and drive right away, it dont move above 50 at all unless i stop in trafic, the engine being cooled to much is not good, but will be ok untill its changed.
Its a pain in the cold weather when you need heat!
Some Alfa site have them for £34 is the cheapist i have seen, e-bay they are £38 plus.
Only got two more weeks untill i can change mine! Good luck