I have the same thing on my V8. The idle would slowly drop, then shudder, and jump back up to 650 or so. Are you seeing it change like that, or is it a steady low RPM?

When I hooked up the duty cycle meter, I can see the display values slowly drop, then when the hiccup happens, the numbers suddenly jump back up and the cycle starts over again, about every 30-40 seconds.

Somebody on another thread said he fixed this by installing a new ECU. Apparently the old one was not responding quickly enough to inputs once the system went into closed loop operation (over 80 C). I have yet to bite that bullet.

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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