TMAllison

What is the general condition of the car? Was it reasonably well maintained? Get a compression test done and if it is not ok, do a leak down or wet test to find out where the problem lies.

If the machine runs well now and then, I would expect this to be an induction or fueling, or both, issue. I wonder if you have some debris (like from rodents) or the typical intake manifold exhaust particulate and oil sludge build up to an extreme level that is interfering with the induction system functioning. I had rodents actually set up home in under the cover over the injectors on my 1998 E300 TurboDiesel. They ate the injector bypass lines and suddenly they all started leaking one day. Had to clean out all the Diesel fuel, and the acorns and other nuts and seeds they had dragged in there.

I am also suspect of your turbo output pressure to fuel system regulation interface. This used to be a device called an ALDA, but it may be something electronic on our cars. Not getting fuel due to a clogged sensor port could also be an intermittent issue.

I doubt a clogged catlyst could perform ok for a few minutes under hard load and then stop, only to work again later at some random interval. I would expect those to get good and hot under load and clean themselves out. Mine blows a bunch of soot the first time I floor the pedal, which I do routinely after it is warmed up.

Good luck and I hope this helps. Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)