Overpressure and overheat at speed are classic symptoms of a bad head gasket, too.
You must correct all the leaks before you can test this, though-- if the system won't hold pressure, it won't cool properly since the coolant will boil in the head, and vapor won't remove heat properly.
Make sure you are cleaning the hose nipples when you replace hoses, too -- if there is much scale or corrosion present, the hose won't seal.
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!