Almost certainly a bad cap unless you also have an overheating problem idling.

Replace the cap -- I had a coolant loss problem in the TE after highway running bad enough to cause the low coolant light to come on. New cap fixed it.

Also verify that the visco clutch is working, and that the aux fan comes on, although you should also be running rather warm to get coolant loss out of a good cap.

If a new cap doesn't fix it, look for a leaking hose or cracked tank.

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!