A 98 GTP has a 4T65E transmission so this needs to be moved to that section. What shift kits are you using and how are they set up? The pcm controls line pressure and shift timing to an extent, so if it is not a very aggressive shift kit you generally cant tell much of a differences. If it was a perfomance kit set on the firmest setting and you didnt notice any differences then the trans is probably just tired but those kits really make firm shifts so something else could be wrong.