I have 1982 Corvette and when you first start out the automatic transmission has a hard bang going from 1st to second gear. Once you have driven several miles the hard shift goes away.
Automatic Transmission 1982 Corvette on 1990 Chevrolet Corvette
Have a shop check out the transmission shift cable, and perform a check of the transmission pressures. You should leave it overnight so they will have it when it is cold so they can duplicate the problem.