Sounds like you may have experienced the dreaded B2 piston failure. Under load, the B2 piston will shatter. I've seen two cases where the car would take a lot of pedal to get it to move in first, but once it was going, the tranny would shift into second and everything was fine. I have heard of several others that are more similar to what you describe, though.

Take a gander at the archive. Search for "B2 piston failure" or some such.

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Gary Thompson
Georgetown, TX
1995 E320
1984 300D