It sounds like a vacuum leak and the computer is adjusting for it. 84's are bad about sucking in the top plate gasket along the back side. Try spraying the seam with alittle WD40 while the engine is running around the seam and see if the idle changes. If it does, you've found your problem.
Well, gang, I think I've resolved my problem. Owner's manual says car "should" run on gasahol (ethanol), but if it doesn't run well your dealer can make certain adjustments. I simply quit using ethanol and tried a tank of Amoco Ultimate (92 octane) and have had no more problems. My next tank will be a lower octane (89 octane) unleaded without alcohol. I'm guessing the hot underhood temperatures when the car sits after a good drive were causing the ethanol to vaporize in the fuel line. We've had some of the highest temps on record the past two weeks here (Iowa) and it was really acting up until I put in the tank of Amoco.
Along the same vein as the issue with the ethanol, is that ethanol will soften the injector winding over time, resulting in high resistance, which will ultimately produce a bad injector.
Remember that your injectors are NOT equipped to run with ethanol, and this type of fuel should NOT be used.
Best to you, c4c5