Hard Shifting Transmission

92 Buick Roadmaster 350 TBI RWD
Just had badly slipping trans rebuilt at 150k miles.
When upshifting, the fresh rebuilt releases torque, then thumps into next gear.
What causes this and is there an adjustment?
The rebuilder says he will look at it next week. He doesn't seem worried. Should we be?
Wendy & John.

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