Sounds like it's "deiseling" - it combustion without spark like a diesel engine. I had the same problem on my 210. It has an anti-dieseling solenoid that cuts off the gas to the idle circuit after the car is shut off. My solenoid wasn't receiving power and so it wasn't working, and when it was very hot it would run on for a few seconds after shutting off the car. But I guess the Z engine doesn't have something like that so I'm not sure how you would fix it. I think it's bad for your engine though. Hopefully one of the suggestions from the other guys will help, but I would lean more toward adjusting the idle than messing with the distributor or other parts of the ignition system. GOod luck