UPDATE: Although I originally thought this could be IACV or ECU reset issues, it seems more consistent with a sticking PCV valve.

Original post below:
I thought I'd post my recent dramas in case it helps anyone. My car is stock and if you have issues with wild rpms while stationary, some of this may help. Also I'd be interested in any expert views of ECU resets because I went into this thinking IACV and came out the other side thinking its ECU gremlins or the PCV (and maybe a bit of all 3).

I'd loaned out my car and it was returned with reports of a CEL which self extinguished after a little while. It was just code 24, based on CEL flashes. That's IACV so I swapped out the valve for a known 'OK' one but the one coming off looked fine. Reset the ECU via the 'proper' under dash plugs method and it cleared the code. The car then drove fine. Put the original IACV back on as it idles at 100 rpm less and I wanted to test it again. Problem solved!

A couple of days and 200 km later (while I wasn't driving again!), it threw another code - but this time the idle was all over the shop. Near stall, 2500 rpm, down to 1000 and back up again, then almost stalling. But fine when driving, this is only on idle. CEL is on. ECU reset does not eliminate the problem this time, but car drives fine apart from being demonically possessed when stationary.

Checked codes with Evoscan and only had stored code 24, stuck IACV indicated. So I swap for a known 'OK' one. No change in behaviour which really freaked me out. Thinking I must have a MAJOR problem I set out to check what I could. I'll spare you the methodical 3 hours of work, suffice to say everything was good and between spraying brake cleaner through the PCV valve, powering down the ECU completely (and removing and replugging the ECU connectors - no evidence of water ingress/condensation as feared), the problem was gone. 850 rpm idle again, smooth as.

I had a suspicion the problem may be the PCV because it seemed to make sense that the PCV could be getting stuck, the IACV works to correct rpm, the PCV unsticks, revs change, IACV tries to alter them, etc, etc. Counting against this, the IACV wasn't that cruddy, I had cleaned it pretty well only a couple of months ago. Last possibility I can think of is that a complete ECU power down is somehow different to the under dash plug ecu reset 'the proper way'. I have seen others suggest that in other forums, and the P1 manual supplement does speak of an IACV reset (separate from an ECU reset) so I guess it's possible that the complete power down I did might have cured some electrical gremlins. Anyway, it seems these symptoms are pretty common, so I add this in the hope it saves someone chasing an IACV. I'll update if the problem returns, but remember the moral of this tale: DON'T LET ANYONE ELSE DRIVE YOUR CAR!