Steve Brotherton is entirely correct (as usual) -- your mixture control system isn't functioning properly, the catalyst is overheating, and cracking from rapid cycles of overheating/cooling.
Get the mixture control system fixed before you fry another one.
Bad ignition will do the same thing, only faster, since you have both extra fuel AND extra oxygen from missfires. Will destroy the catalyst pronto.
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!