Portugal here! :-) Mine, heats progressively and steadily, and when it reaches 90º spot on, it stays there - This happens all year long, not matter what.
Mine is a new one. Because before, it would come down from 90º in the city, to 70 in the highway.
Since I replaced it, it immediately stooped that behaviour.
That's why I find it so strange that you claim its natural to have it going up and down!
Mine goes up, and it only comes down with the engine turned off.
Ps- By the way I'm not really interested in what the real temperature value is in my car here, or yours "up there". As someone said it's not an exact science. What I DO take notice is the behaviour of the "needle" in the gauge, which if too erratic may indicate some problem PERHAPS with thermostat :-)
Again, not real mechanic on this side of the screen