97 E420, engine runs fine when moving, but at idle the rpm is very unstable and keep dropping to 400rpm or so and wants to stall. it actually never dies but the computer will shoot the rpm back up.

can anyone please help.. is this a symptom of 1 part (CPS?) going bad, or something bigger..??

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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