TDI engine temperature too low

[bobiscuit]

I have a 2012 Passat 2.0 TDI (140) DSG

Sometimes the engine temperature stays low for an entire drive. Regardless of speed - it has happened while sitting in traffic for hours or while sitting at 70mph.

When this is happening, the engine temperature gauge will go up and down, but never reach 90, and the oil temperature reading fluctuates from around 60 to 80. Also the air into the car doesn't get hot enough, but now we're out of winter that doesn't matter too much.

But then other days it will be 100% fine - it rises up to 90 within 10 minutes of a cold start and stays there the whole day - and the oil temperature sits between 90 and 100.

I've taken it to a couple of VW dealers but obviously it behaves perfectly on those days!

I remember reading something about a thermostat that circulates coolant to the DSG box, which can cause these kinds of symptoms?