I had the same symptom when I forgot to plug the idle air control valve back in. As you can imagine, it didn't fix itself. I also had to sort out a similar problem when I got the car, but instead idle would fall when hot.
I doubt there is a type of bad gas that works better with a hot engine.
First, do some visual SHO checks. Pull out all the spark plug wires and see that they are bone dry, not oily or wet. Otherwise, you might need to use the wire as a swizzle stick in a glass of gasoline cocktail to rinse out the oil and water, and also clean the plug wells well.
Then I would do the idle re-learn procedure. Disconnect the (fully-charged) battery with the lights switched on for 10 minutes to discharge the computer's memory. Then on a cold engine, pause at key-on for a few seconds to prime the fuel pressure, then crank and start the car without touching the gas pedal; resist the urge to touch the pedal even up to 10 seconds of cranking. Make the car figure out how to idle. If it starts, let it run for two more minutes without touching the pedal, and then another minute with all the accessories on and turning the steering wheel before you go for a short drive.
Then the most likely suspect is indeed the ISC valve (IAC is actually the name of the secondary runner's intake air control valve in 1989), the cylinder mounted just across the driver side of the intake from the throttle body. You can clean first: Unplug the power connector, remove its two Allen-head bolts (without losing the gasket that will drop out the bottom), and then give the insides a soak of carb cleaner and dump out and spray rinse again. Try the procedure again.
Then the next before replacing that valve might an unlikely old-gas scenario - the charcoal vapor canister. This collects fuel tank vapors, and is only supposed to purge them while driving. The solenoid that operates it can fail stuck-open, and the container might be saturated with fumes. This can be eliminated by pinching the small vacuum tube that comes out the front of the intake near the ISC (and heads down the front of the engine compartment) with vice-grips.
You can read the codes. Key-On-Engine-Off will first blink the codes for any controls or sensor that diagnose out of spec.
Still doesn't idle right? Then you can report back as you've got challenges, doing maintenance like replacing the fuel filter and checking rail pressures, checking for vacuum leaks, spark ignition quality. California EGR stuck open also will make for poor idle. Rodents been eating on the vacuum hoses, wires, or gas lines...? And then a full diagnosis of engine controls after you get a full tank of super.
