OR IT COULD BE...…….
Someone didn't do the technical service bulletin (TSB) for the carburetor back in the day and you may just now be having that issue?
Obviously if you're having issues now and not way back in 86, it no longer applies anyway, but the same thing sorta happened to my 86 Monte SS at about 1500 miles or so IIRC. Except the idle was acting like it was stuck on the high cam of the choke at hot idle. The claim is that the primary rods have burrs on them from the factory making them hang up. At least that's what they told me at the dealership when they did it. It was under warranty so they were going to fix it. Bulletin said to remove carb top, replace rods with a new rod kit and air horn gasket kit, yadda yadda....dealership just plopped on a new carburetor, test drove it, and sent me on my way. Anyway, it didn't idle high again after that. Service dude said mine was the third one in two weeks to come in with the same issue.