The usual cause for this behavior is a slipping belt. Check the side with the grooves -- if there are cracks or pieces missing from the ridges, you need a new belt, and likely the problem will go away.

Sadly, you will also likely need a new belt tensioner too, a royal PITA.

Peter

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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!