Bent rod syndrome is accompanied by a terrible clank, sudden oil consumption, and usually blue smoke/lousy performance.
Intermittant power loss with black smoke means either valve problems or, more likely, an injector sticking open. Run some diesel injector cleaner through the tank and see if that helps. If not, time to see someone about rebuilding (or at least checking) in the injectors.
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!