Get a new pressure cap. I had the same problem on my 300TE when I bought it -- PO said "it had an overheating problem with the AC on but they fixed it" -- typical "soccer mom" didn't have a clue as to what they did.

Turned out the AC was discharged (leak at the expansion vavle), and when I charge it, it overheats on the highway. Would blow several quarts of coolant out when I stopped after a hot run. Put a new pressure cap on, and it only gets too warm in traffic now (need to install the new visco clutch this spring).

You could also have a bad thermostat, in combination with the bad pressure cap the coolant will boil in the head and prevent circulation.

Thermostats are cheap, and easy to replace, so I'd do that too.

Peter

__________________
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!