I agree with Jim -- big leak in the climate control system (can simply be a hose off, for that matter) that lowers the vac when the climate control is on.
Flex disk is a 60,000 mile maintenance item on these cars, by and large -- not in the book, but Hans says 60,000 is about it. He tends to replace them at the 60,000 mile service, nearly always going by then.
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!