I have been troubleshooting a similar issue to yours. Same car, 97 GrandPrix SE with 3.8. The car stalls out when letting off the gas too fast. I replaced the TPS which didn't help the problem. Then replaced the Idle Air Control Valve. This made a big difference, but didn't totally clear it up. It will still stall about 10% of the time (when trying to make it stall). I also cleaned the MAF with some precision electronics cleaner.
While changing those valves, I noticed the soft rubber vacuum T that fits on the throttle body is cracked. This T connects hard plactic vacuum lines to the fuel injector regulator? to the throttle body and then goes down to something? I've hit up all the chain autoparts places, but no one carries this T. Looking like a dealer part.
