Watch it very closely indeed ~ you may find after the next really hard , hot run , the coolant turns slightly cloudy ~ this will be the accumulated silt and sediment dumping into the coolant and indicates you're not done yet .

It took a long time to fully clean two of my five old Mercedes .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

As has been written before the thermostat has nothing to do with an engine that is running hot unless the tstat is stuck closed. The thermostat's purpose is to control the engine warm up after being started from cold.

The original poster did not post his speed when the engine ran warm. Typically diesel engines will run warmer when the speed is 75 mph on a consistent basis.

Around 70mph+ on the highway the temp does come up a little bit, even in the colder months. But running near 100 seems wrong.
When I was having the temp climb over 100 was steep grades for several miles doing about 50mph. Turning off the AC and slowing to 40mph held the temp stable at 100.
Unfortunately, where I live is almost an hour from any speed limit over 55 with no long steep grades. Just driving around the temp is lower than normal, though. I have high hopes it's solved.

I know the thermostat likely isn't the issue, but I don't know how old it is, so replacing it is probably a good idea from a preventative maintenance standpoint.

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1984 300TD -- summer daily driver
Many others that aren't Mercedes...

Perhaps you have too much antifreeze in it. Once in a while this happens where there is too much antifreeze and very little water to help with the heat transfer.
This is easy to test. Drain about 3/4 of a gallon and replace it with distilled water. Drive around and put a load on it. If it's still an issue, take a look at anything from the thermostat not opening fully, radiator clogged internally, radiator clogged externally, head gasket (if loosing coolant only with a load on it) and etc.

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1983 123.133 California
- GreaseCar Veg System

It's not losing coolant (since I replaced the leaking WP...), or showing any other signs of a headgasket failure that I've ever seen. I tested the antifreeze before winter, and it tested fine, I guess it could be a bit rich, though... I don't wanna risk going too watery or it'll freeze when it's -35 outside...
I'm just gonna have to wait and see after the recent repairs. It's been pretty hot lately, but I generally drive my beater old subaru for work because I don't like throwing tools and dirty crap in the mercedes.

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1984 300TD -- summer daily driver
Many others that aren't Mercedes...

Normally overheating problems that occur at higher speed are due to air flow through the radiator, a plugged radiator, a defective water pump, loose belts, or a faulty fan.