Pull the monovalve (two screws, in plastic, so be careful to get back into the existing thread going back in) and look at the rubber diaphram. If it is wrinkled or has sharp creases in it, replace it, it's either leaking or not closing properly, and hot coolant is leaking past the valve to the heater core.
You should be getting cooling pretty fast -- both the 300 s blow cool air right away, but it does take several minutes at 45-50 mph for it to get cold.
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!