Mine does that too. Sometimes it misses, sometimes it wavers up and down between 500 and 800 rpm. I read of someone else having the unsteady idle problem, and the culprit was a bad ECU.

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1985 380SE Blue/Blue - 230,000 miles
2012 Subaru Forester 5-speed
2005 Toyota Sienna
2004 Chrysler Sebring convertible
1999 Toyota Tacoma