car: 1997 E420, M119 motor, 107k miles

Car starts, runs, and drives great. 25.8MPG on the highway. Smooth, fast acceleration, no abnormal noises. Spark plugs, connectors, serpentine belt replaced recently. No Check Engine or malfunction lights on.

Problem: When driving around and coming to a complete stop at a traffic light, the rpm would drop to just below 500rpm and engine will almost stall. Car will shake, but the computer will shoot the rpm back up to compensate. This will go on every 5-10 seconds until I start driving again. Basically the RPM will never stay the same but always move around at idle.

After searching the forums, it could be one of many reasons, such as

1. bad spark plugs
2. bad Mass Air Meter
3. bad engine mounts
4. bad oxygen sensors (which ones? all 4?)
5. vacuum/air leak on vacuum lines
6. clogged fuel filter
7. bad CPS sensor
8. bad fuel pressure regulator


Now the last thing I want to do is start blindly throwing new parts on her and keep spending $$. But because there is no CEL on, pinpointing where the problem is tough.

Any ideas/suggestions??

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Current cars:
2001 MB E55 AMG
2003 Honda S2000
2000 Honda CRV

Gone but not forgotten:
2004 BMW 325i 5sp sport
1999 BMW 540i sport
1997 MB E420
1995 BMW M3 coupe