93 Deville trans slipping
 

1993 Cadillac DeVille 4.9L FWD 118,000 miles.
Transmission violently grabs and slips (surges) only at part-throttle on slight incline, most noticeable around 35 mph. Otherwise no noticeable slippage. No computer error codes. Dealer replaced transmission for $4300 and car still has exact same problem! So what else could be causing this problem?