You might want to pull the rubber tube off the modulator to see if there's any transmission fluid leaking past the diaphragm. Mine was bad, causing the 2-3 slip as you previously described; so, I don't know if any amount of adjustment would have helped me. It only takes a minute to do if you've got some long needle nose pliers. It might be a cheap fix.

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