I have a 97 Olds 88 ls that shifts into 4th gear early and it surges about 200 rpms when going down the road. It sometimes seems like the torque converter clutch doesn’t know when to stay locked up or not. When the car is cold it seems to shift fine, but when it gets up to temp is when it starts to happen.
Have you checked your transmission fluid level lately? When was the last time you had it serviced, including new filter and such?
The level is fine and the fluid smell fine. As for when was the last time it was changed I don’t know. I just got the car. I should probably change it. The problem stops for a moment when your going down the high way and you take your foot off the gas for a second and the converter unlock. When it locks back up the problem comes back.