Toyota - Corolla :: 1990 - Idle Speed Increased For Some Reason

Over the past month I've notice the idle speed increasing. Yesterday I had to turn the idle adjustment screw to lower the idle speed, as the high idle speed was making it difficult to drive in stop and go traffic. Especially if I took my foot off the gas while in gear, it would jerk rather than slowing down smoothly as it used to. I think the jerking was related to how the ECM stops the injectors completely when slowing down in gear, to improve mpg, then before the rpms gets too low, fire the injectors back up again. Anyway, lowering the idle speed with the adjustment screw fixed the jerking problem, for now.

But in 20 years, I've never had this occur before. The only time I've fiddled with the idle adjustment screw was during tune-ups, and then only had to adjust it 5-15 degrees. This time I had to turn it in 45 degrees. I expect there is unwanted air getting into the intake manifold somehow. And it will probably get worse. Here's some ideas -- in order of most likely to least likely -- I've had what could be causing it.

Throttle valve sticking in throttle body and needs a cleaning.
Throttle linkage needs a lube.
Canister purge valve is open during idle and allowing fumes from the canister into the intake manifold, when it shouldn't be.
Throttle position switch has failed.
Vacuum hose or device leaking.
IAC valve (part of throttle body in this car) is sticking.
ECM is bumping the idle speed through the vacuum controlled actuator (VSD). Unlikely, b/c the idle speed still increases when I turn the headlights on or I turn the steering wheel, both of which use this mechanism (I think).
Thermostat -- unlikely as it happens even when the coolant has reached 195 deg F (when the fan turns on).