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Nate, here's my latest installment on 16-valve issues related to your problems. Your additional info keeps pointing to two distinct areas. [FYI: ECT is the "Engine Coolant Temp" sensor]
As for stalling:
A cold high idle immediately followed by a step down to normal idle is indeed the usual sign of a sticky IAC valve. Engine vibration allows it to close. If you give the IAC valve a good tap using the end of a rod or stick and the idle changes then that's also a sign that the valve is sticky. Flush the valve section throughly with solvent or carb cleaner. Shoot some WD-40 inside around the motor end of the cylindrical valve itself then work the spring loaded valve open and closed by twisting the valve body in your hand or using something non-scratching like a small wood stick. If it still misbehaves then the motor section is likely worn out, but it's not really cleanable nor serviceable. This is basic maintenance and needs to be done periodically, like with every other flame trap cleaning.
As for intermittent rough/poor running:
If the engine sometimes runs not too bad and then starts misbehaving during/after damp weather then you should be very suspicious of a marginal electrical connection somewhere in the fuel/ignition management system.
Always a good place to start is a through electrical tune up. Things to check are the AMM connector, the ECT (temp) connector, the injector connectors, the O2 sensor connector, the rpm sensor connector and the grey connector blocks (all at or near the right strut tower) plus the LH ECU connector (be careful) plus all the ground points (esp. those on the intake manifold and on the ground rail behind the ECU). The EZK ICU (above the gas pedal) is generally not exposed to moisture and likely doesn't need attention.
The proper way to deal with this is to open up the connectors, use a safe spray contact cleaner, apply dielectric grease and re-connect. On ground contacts, OxGard (avail. as anti-corrosion paste for aluminum household wiring) is the preferred sealing agent. The O2 sensor gets too hot and should be cleaned, but not coated. Make sure all pins at the grey connector are pushed together from the back for a solid connection. Make sure all rubber boots are clean and waterproof. You can pack the boots with dielectric grease to keep water out. At the very least coat inside to seal both ends of the boots. You should fix any damaged rubber boots with RTV sealent. Even if this doesn't cure your problem now, it will certainly help keep problems away in the future.
Now, the more I think about it the more I'm starting to suspect your fuel system relay (aka. radio suppression relay) is the culprit. A marginal relay never enjoys moisture. This often overlooked little beast supplies power to the injectors and can be found on your rad coolant bottle. It's certainly known for causing horrid running if it fails to make a good internal contact and starve the engine of fuel. The engine can suddenly stumble and just as suddenly recover. Try re-soldering it (Fosterizing as we say around here) before replacing it. Even if the solder joints look good, do it anyway, it's hard to see the microscopic heat stress cracks.
Still on my hit list are:
A weak rpm sensor, if it's the original style. It's in the top of the transmision bell housing just below the distributor. A weak signal under poor electrical conditions would certainly cause rough running. I don't think they fail as much as they just keep getting weaker and weaker.
A bad ECT sensor is always another thought, but that usually will typically show up as a drastically higher idle and/or smoke out the back end and excessive fuel consumption due to rich running). BTW if anyone has been working in that area, there is always the possibility that the ECT connector got swapped with the knock sensor connector (without checking, I'm pretty sure those are the two identical ones). The engine will run like this, but not happily. If you suspect this then you need to peel the connector boots and check the wire colors against your diagram.
You keep suspecting your O2 sensor, but as I recall it seemed to pass the resistance checks and nothing you've said really seems to point to it. The signal from the O2 sensor isn't even used until the engine has reached operating temperature. That doesn't mean it's not the problem, it's just not as likely and not overly common for that type of O2 sensor to fail.
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