92 Buick RoadmasterTransmission

1992 Buick Roadmaster 350 TBI:
The transmission began slipping badly at 160k miles. We okayed a recommended rebuild by a transmission shop.
The rebuilt unit acts funny; when upshifting it thumps, then lets go for about 1/2 second, passengers are thrown forward as the car slows, then the next gear engages smoothly.
The transmission shop has tried repeatedly to adjust it (it was much worse when delivered) and now will do no more, saying the performance is within the normal operating performance range for this type of transmission. They say we have nothing to worry about because there is a one-year warranty against transmission failure.
Even if the tranny lasts forever we will never get used to feeling the thumping and being thrown forward with every shift.
Does anyone have any suggestions for correcting this unwanted performance, or for making the trans shop deliver a better quality unit? Do other Buick owners accept this performance?
Thanks for any help you can offer.
Wendy & John. --------------------------------------------------------
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