Engine running temperature

[Dean_]

Hi, I have a 2006 S max 1.8 tdci but ever since I have had it the engine temperature sits around 70 on the gauge? It does increase to 90 when I push it hard up a hill then drops off again after. This shows all the signs of a faulty thermostat. This is bow the problem, so far I have fitted 4 new thermostats and seals ( 2 genuine, 2 non genuine), new genuine thermostat housing, 2 new head temp sensors, new water pump and belt.

I am now stumped on what it can be. I do get heat inside the cabin so engine is getting warm but not sure if it's full temperature. I have read about the focus having dash cluster issues which affects the temp gauge but nothing related to the s max?? 

Any help will be greatly appreciated.