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When you release the throttle on a DME equipped car the injectors stop injecting fuel (to save fuel). At a predetermined RPM (I think it's around 1000 or so) the injectors fire back up in order to slow down the RPM drop, and then the engine will settle into a normal idle.
Why the engine would drop all the way to 500 or so before the injectors fire up again is beyond me. However, I had a similar situation arise. After a lot of fiddling with various components, I finally decided to bring the car to the Porsche shop I use and asked them to adjust the idle mixture and idle speed, both of which are interrelated and crucial for a DME controlled engine. After they did that, the car ran perfectly, and has ever since.
You might check your ISV (Idle Speed Valve) for proper operation before you take it to the shop though.
Do a search for this problem on this forum. There is a lot of good info to be found. Try something like "erratic idle".
Good luck.
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