"The engine still would periodically lose power while under load, in some instances occuring every 15 seconds or so, until I gassed up and found that the gas cap had been off all this time."

I seriously doubt this is the root of the surging idle. This is not a modern car where the absence of vacuum in the fuel tan will trip an CEL. Couple of questions:

How full was the tank before you filled it up?
Have you removed and clean the IAC mentioned earlier?
Does the surging (loss of power as you describe it) only happen at idle?

You idle is controlled almost entirely by the IAC. It is "turned on" by the throttle switch so now it is probably doing its job as well as it can after the switch adjustment.

The reason I ask how full the tank was is that the lift pump in the tank has a notorious habit of failing about every 80K miles or so. More importantly, the rubber line between the in-tank pump and its fuel line is often degraded so it sucks air an 1/3-1/4 tank. That tiny section of hose can fail in as little as 10K miles if it was replaced with hose not rated for submersion in fuel the last time the owner replaced the in-tank pump.

I cannot remember if the O2 sensor plays any role in the LH2.2(?) system that your car has but that would be a distant possibility even if it does.