It may just be that slightly lumpy idle these engines have--have you tried disconnecting the battery for 5-7 minutes to clear the KAM (Keep Alive Memory)? The KAM is used to store learned values like idle air trim, long term fuel trims, etc.

Clearing it will force the PCM to relearn those values. I do this between what passes for seasons down here. We have 2-1/2 with one that repeats--nice, hot, friggin' hot, and then another hot before nice rolls around again. Nice is the 1/2 season...

 

I disconnected the batter for about a half an hour after doing the IAC. Not sure if it would be related, but every once in a while the car has a bit of trouble starting. It'll crank, and I'm pretty sure it does actually start, but it's a very anemic start, like the RPM's are extremely low, then it dies. I'm not sure though since it's fairly infrequent and happens pretty fast, usually when I'm not paying close attention. This all happens over the course of a second or less. Then the next key turn she'll fire right up.

 

Check the battery voltage while cranking the engine, it has to be more than 10.5V while cranking (preferably higher) or all sorts of problems from weak spark to improper sensor inputs, and even PCM glitches can occur.

If it is near or less than 10.5V while cranking then have a 1/2 CCA (1/2 Cold Cranking Amperes) load test run on it--at 70° battery temperature it should hold 9.6V or better for 15 seconds.

 

On that note, do you run one weight of oil year round, or do you change it for the "seasons" down here? Either way, what would you recommend running? It's close to oil change time here soon, and wondering what I should run.
(P.S. I love the very true way you summed up our "seasons" down here.

Sorry, OP, don't want to hijack your thread.

 

I run 10W-30 synthetic (usually Pennzoil) in the Mustang (115k miles), year round, 10W has fine cold cranking characteristics down to -15°F and is good to 120°+F on 'tother end.

In the wife's Toyota Highlander with 175k miles on it I run Valvoline Maxlife synthetic blend 10W-40--also year round.

In my bike (17k miles) I run Valvoline 20W-50 4-stroke motorcycle oil (dino) per Suzuki's recommendation for 14°F and up.

All of the above are changed every 2500 to 3000 miles...