The fan clutch can be re-filled, there is a thread here somewhere (possibly in the DIYs).

Start at the beginning. You drove your car, and it went to the 100C line, that's no big deal. Shouldn't run that warm on the freeway, but not dangerous, ... so what could be causing it?

First, if there are no other symptoms of cooling system problems, consider replacing the temp-sensor. It is the green (or blue) 3-wire sensor just forward of the #1 injector, it turns the aux-fan on (at 105C?) directly, and feeds temp info to your gauge. It might be bad and throwing your gauge off. The new one is a lower temp cut-in for the fan so that's all good, and IIRC it's only around $20 or so new on-line.

The aux fan. Should run if you power it directly, if it didn't then the fan is one of your problems. Does the electric (aux) fan rotate freely by hand? Powering the fan resistor should give you low speed on one side, high speed on the other. The Freon pressure turns it on low. High speed is run from the green (or blue) coolant temp sensor mentioned above, short across the two-wire connector to run the fan on high.

Next is the clutch fan. At that 100*C mark, rev the engine. Do you hear the roar of the fan? If not, then either your fan clutch is bad, or the temp gauge is reading high (covered above). The fan clutch should also be engaged cold, when you start the car in the morning, should roar on first start/rev of the engine. If not, it is bad. Also check for play in the fan clutch, if that sucker is bad they have a way of eating expensive radiators.
The clutch fan should keep the car below 100C (normal driving / normal temps), and without it you will see elevated temps. If the aux fans are working then it should come down when it hits 105C but the clutch fan is necessary even while driving as the aerodynamics of the 124 don't shove as much ram air through the radiator as the old days.

If these things are fine, check for hot and cold spots on the radiator. If it is hot in some areas and cool in others, likely there is a restriction in the radiator, bad thermostat, or bad water pump. Using the wrong coolant can eat the water pump so that's a possibility over the years. Generally the radiator shouldn't vary much in temp from one side to the other, significant temp variations indicate blocked tubes.

How many miles are on the car?

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