Replace your "radiator" cap -- it's on the coolant tank.
The one on my 300TE was bad - previous owner told me it "overheated with the AC on", was "fixed", but couldn't remember how. AC was discharged, so I switched it to 134a.
My sister's Volvo blew up on the way here from Florida, and I couldn't get it fixed in time for her to continue up to Cleveland, so I lent her the MB. Sure enough, when she pulled off the highway, the low coolant light came on.
I fixed her car and drove to Cleveland, then drove the MB back home. It was fine until I got to Cinncinatti, where I stopped at a rest stop. Blew coolant all over the parking lot.
Spent a couple hours driving around and stopping at every parts store I could find, and finally found a place that had a radiator cap, and sure enough, with a hot engine (off the road for about three minutes), there was no pressure in the coolant tank.
Popped the new cap on, and problem was gone. Temp goes up to about 100C in traffic, comes down pretty quick when back up to speed, just like it should. I have a new visco clutch I've not gotten installed as I think the old one is bad, but haven't had a single problem with spilled coolant since.
Pretty cheap fix -- $5.00 for the aftermarket, $13 or so for the MB cap I bought after I got home.
If the cooling system doesn't pressurize, you will lose efficiency and the coolant will boil in the head when you slow down after a highway run.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!