I have the four speed transmission, and mine thunks the same way, real hard, from 2 to 1, when I'm decelerating to go around a corner, than hit the accelerator real hard. The car goes "bam" into first gear.
If I accelerate gradually after slowing down, the tranny shifts properly.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".