What is the base idle speed on your engine?
Ok - looks like the MP conversion unit because it's too shiney to be an original one. That's the up-side. The down side is to really look at it you'll need to remove it. That's not a big deal though. Take off the distributor cap, and use a Sharpie marker to mark the position of the ignition rotor's tip on the edge of the housing. Then make a single line that goes from the housing to the block on the base of the distributor. Don't make the mark where the hold down bracket is. Then unbolt the hold down, unplug the wire connection, and pull out the distributor carefully.
Next - look down a the plate that sits under the rotor and the reluctor (the 8 pointed "star" thing). It will have a couple holes in it about 3/8" or larger. Shine a light down there and you'll see the advance weights and springs. You may need to spin the shaft slowly to see both weights and both springs. You want to look at the springs. There should be one heavier than the other - visibly thicker and more coils in it. Also, you'll want to spin the shaft until you see one of the slots of the advance plate through one of those holes. When it's visible, hold the shaft, and spin the reluctor counter-clockwise lightly. It should move 10-30° and you should see the pin move in the slot. Check both weights that way. Snap a couple clear shots of the springs through those holes and post them. My impression is you have two very light springs in there rather than one light and one medium. The engine is turning fast enough that it's advancing when it should not be. When you shut the car off, the springs pull the advance out, and the rpm drops to base idle. If it's just a spring you need you can buy them, or I can send you one.
