Long time listener, first time caller here...
My car (2001.5 Passat wagon, 1.8T) just had its 105k service (new water pump, timing belt, tensioners) last week at the dealership. I also asked them to replace the thermostat and flush the coolant, as the temperature gauge had read low all winter and the heat didn't really work, suggesting the thermostat was stuck open.
I've driven the car over 100 miles since, and the temp gauge still reads low sometimes. It's close to 190F after ~10min of highway driving in 60F weather, and sometimes gets to 190F after ~15min of city driving. But this morning it stayed at the low end of the scale after 10min of highway driving in 30F weather.
Does this still sound like a thermostat problem? Or do you suppose the car's computer still thinks it hasn't been replaced, and maybe disconnecting the battery for 5 minutes (to reset all the "learned" parameters to default settings) might work?