Hi, just joined and have been serching for an answer to my issue. I have a 2000 Boni se. It has 108k on it. I notice when driving under light throttle the trans shifts sloppy and hard. When I make a hard turn it shutters. When I gun it or get on the highway it is perfect. The fluid was mostly pink when I bought it and I changed it with no difference. Tried the seafoam to and still the same. Any thoughts?

Also is it normal for the car to feel like it is riding with no shocks over bumpy road. On the highway and over larger bumps it feels great. The small multiple bumps feels like crap.

Does it shift hard on all gears? Had a Grand Prix here a while back that had harsh shifting. I dropped the pan and remove the shift modulator. Opened that up and scuffed up the cylinder walls just a little. Put it back together and it shifted fine. I'll have to see if I can find that thread.

It shifts hard in all gears. Crazy thing is if I shut the car off and start it back up it runs good for a while then starts shifting hard again... It also shifts great when it is cold.

This write up is for making and installing a shift kit. All the steps to scuff up the cylinder walls are here. The only parts you can skip is adding the spacers, unless you want to.
https://www.gmforum.com/performance-137/making-installing-shift-kit-4t65-hd-transmission-278802/

Scan the computer and see if a code P1811 comes up. If so, you will need a pressure control solenoid.
If you don't see a P1811, hook up a 300 lb pressure gauge to your line pressure port and drive the car with it connected to see if your pressure is within spec. The should not be major fluctuation in the gauge. The pressure tap is right on top of the trans and is easy to find,

Thanks for all the info!! I will try all the suggestions.