When in warm weather or after driving a lot of stop and go, my 1996 Pontiac Grand Prix doen not shift properly. The check engine light comes on and it wont shift up to the next gear.

4T65E tranny? Sounds like the car is going into "limp home" mode when hot. See if you can get the OBD code, Pep Boys will read it for free and maybe even tell you what's going on. "Limp" mode puts the tranny in 3rd gear, which translates into very slow acceleration, engine light on, and a top speed of about 50. You may have gotten some water into a connection, and there's a ton of them under the hood. Suspect the Powertrain Control Module (PCM). Good luck hope that helps.