Regarding the thermostat - I also have problem with not getting to 90 degrees, than I have replaced the thermostat with a genuine VAG one ( but it might be a false one) - and the effect was worse than before i this job.
Not giving up I have bought another one (manufactured by Calorstat), first tried it in a pot (opening and closing temperatures) - which were better than with my "old" one. I have installed it to my car and the result is great - it takes about 3-4km of driving to get the engine to 90 degrees, but most of all, when it get to this temperature, it is not dropping (as it was before).
Afterwards I checked the new "OEM" one and after it open, it is not closing back at all...
I guess that the conclusion is that it is good to check the thermostat operation before installation in the car.