There is something seriously wrong with either the ignition system, fuel system, or installation. Cat should last MUCH longer than 200 miles!
I'd figure the shop would have fixed this by now, they are going broke on warrenty work!
Check ignition system -- it may still be bad after the repair. You need to check the fuel mixture at idle and under load, too -- going intermittantly rich will fry the converter, causing either meltdown or thermally induced cracking.
And check the hangers and basic exhaust intallation -- if the pipes are banging because they are improperly installed or the hangers are bad, the cat will evenually break up from impact damage.
My money would be on the ignition.
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!