Before I would let anyone start swapping injectors, I would build the code reader and pull the codes. I didn't read if you changed the fuel filter. If it's clogged, it could cause the engine to lean out and you could experience similar problems under a load.

You can either spend a lot of time and money throwing parts at the car, hoping that you get lucky, or you can pay to have a good MB tech trouble shoot it for you. Many of us here prefer not to take our cars to a tech because it's kind of like admitting defeat (at least that's how I feel).

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles