1991 Cutlass Supreme Idling Problem

I have a 1991 Cutlass Supreme with the 3.1 L engine. The problem I have is sometimes when I'm going driving and start to slow down, the engine idle remains high until I stop then the drops too low and starts chugging and running too rich. When I take my foot off the brake and hit the gas, it idle jumps back up again and runs high. I've check the torque converter by unplugging the lockup at the side of the tranny. I've replace the MAP sensor and changed out both the TPS and Idle control valve with ones that worked on a different car. The only thing left that I was thinking about was the ECU but I don't want to continue to throw money after money at this. It seems that too much gas is going into the intake and that is why it is running so rich therefore causing the idle to be racing. I can neutral it while going down the road and the engine will still race....I will turn the car off still coasting....wait a few seconds....restart the car and 70% of the time the idle will cooperate and the car will run fine after that.....until the next time I let it park it and run it againl. any ideas out there?

Welcome to AF. Check fuel pressure, check for raw fuel in the vacuum hose to the fp regulator, check for proper vacuum to the map, for starters. Scan it and check for fuel trims out of whack. Check the coolant temp sensor readings on the scanner, try tap testing the pcm see if the engine stumbles. Disconnect the egr for test purposes, clean the iac and throttle body.