I just described this procedure aux fan removal) to slowmoe about 4 or 5 posts up on this thread (the long post). If you decide to do it, it wouldn't be the worst idea in the world to go ahead and replace the small fan drive belt as a maintenance item.
You could TRY the thermostat, but don't blame me if it was a waste of time and money, or you could remove and check it, but for the problem you're having, it would all be pretty subjective, in other words you're looking for a very minor variance in the thermostat operation. I assume you'd be looking for the tstat to open just a little bit later than normal, and maybe not as "open" as it should be. You'd almost have to test yours and a new one at the same time for comparison. Or hell, just throw a part at the problem, you wouldn't be the first, but remember the caveat I listed in the beginning of this paragraph.
Other problems it could be....well, that's where it helps to have a guy actually working on the vehicle, because maybe there is some oddball thing going on I can't think of.
I'll assume it hasn't been in a wreck where the front was damaged and *GASP* some fine young bodyshop professional put something back together wrong, ie aux fans wired reversed, or engine fan put on backwards, both of which I have personally seen, but maybe not on a 210. (They're real famous for messing up the turn signal and headlight washer wiring on 124 chassis, easy to do, but EASY to check when you're through).
Ummm, it sounds like the cooling system has been well cared for, but another possiblity is the radiator CORE plugging up (the tubes inside the radiator), UNLESS you had a prior owner or shop that really got "in to" this radiator stop-leak crap as a maintenance item, and now THAT's plugging the core. But outside of that scenario, I doubt it's the core plugging up, but something to chew on.
How high is the temp actually going? The "rate" at which it heats up to tell you the truth wouldn't concern me as much as how high it's going. You can ask around the forums and see if someone would check the rate at which theirs reaches normal engine temp, and what normal engine temps owners are seeing on theirs.
I'd assume 80 to 90C to be normal.
Possiblity exists also I suppose that it's just an instrumentation problem, but that's initially what they thought about Apollo 13 too......
Gilly