Wow, I don't know how I missed this thread before. Let me first say this (although you probably already know, but in case you don't): you idle is controlled entirely by the temperature of your coolant. There's a cold idle valve that is made like a thermostat internally, then the IACV that you have messed with, and in my experience cleaning those very rarely actually improves it, they go bad internally and no amount of cleaning will fix it. So you can chase the inputs to the ECU like the O2 sensors, coolant temperature sensors (One at thermostat and one at upper radiator hose port on the head) and the IAT sensor (not sure if the prelude even has one honestly)- but really I suspect the IACV is bad. I know it's not what you want to hear since they are expensive as hell, but personally, if it were my car, I would look into that. As others have mentioned, compression in the coolant passages via head gasket can create that, but the engine would run hotter, not colder. Here's one thing I've had happen to me. The condenser fan went bad on an accord. It started idling bad, and boiling over via the radiator cap into the overflow. Turns out the radiator cap was broken too, so it was two thing contributing. The engine got way hotter than normal, and when it did, it boiled over even easier because the cap was bad. If you have a bad cap, it could let coolant out, the temperature sensor can read low in the absence of coolant flowing across it, and since the idle controls need coolant passing through them, in the absence of that they could read lower (while your engine may in fact be hotter). Highly unlikely, but just some food for thought on the interrelation of the different systems on the car. If it were my car I'd be buying an IACV since they don't last forever on Hondas (far from it) anyway, and if by chance that didn't fix it I'd use a timing light and check out the ignition timing (after tuning it up, plugs, wires, cap, rotor. If it persisted, I'd then buy an intake manifold gasket set and all the vacuum lines and redo the whole intake. Also, you could use a compression gauge and see if all your cylinders still have decent range compression. Since it's an H22 I'm guessing you drive it like one, and despite popular opinion H22's aren't totally bullet proof if you're running the shit out of it.
That's just my two cents worth, it sounds like you're pretty damn smart though, you've already started down the right road, you'll get to the bottom of it eventually, it's just persistence man.
