For what it's worth, I had a similar problem years ago where the engine almost stalled when putting it in reverse. It turned out to be a bad neutral safety switch. But if it's only happening with the a/c on, it sounds like Jim's suggestion is correct.

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