I have a '95 R-code S351 that had the same problem and it too made the car annoying to drive. Start with the basics and go from there before you start replacing/adding components. First, you want to understand the reason that the idle hunts. The reason for that is usually because the exhaust readings (O2 sensors) does not correspond with the air inlet readings of the mass air sensor and the exhaust readings will either be too rich or too lean. The engine's computer will "hunt" up and down it's idle setting parameters to try and match the mass air flow sensor's air intake readings to the O2 sensor exhaust readings, hence the "hunting" idle. Unmetered air (vacuum leaks) and/or incorrect mass air flow sensor signals are common problems. The "air adjuster" mentioned by Footnfan will tweak the mass air flow sensor's signal and can compensate for an incorrectly functioning or calibrated mass air sensor. However, if your mass air flow sensor is functioning properly, then it won't help you.
First, check for vacuum leaks. Any vacuum leak downstream of the mass air flow sensor will create a hunting idle. I've been able to correct numerous hunting idles by finding cracked rubber hoses and torn intake boots.
Second, try removing and carefully cleaning the mass air sensor's sensor element. Dirt build-up on the sensor element will create incorrect sensor readings. This is especially true if you have an oiled filter element (such as a
K&N filter). Sometimes the oil used on the filter will contaminate the mass air flow sensor element (especially a filter that has had too much oil applied to it) and create false readings.
On my particular S351 I found a cracked vacuum hose and a broken PCV valve. I also replaced the idle air control (IAC) solenoid with one for a 351 Lightning pick-up. I also installed the Air Adjuster because the mass air flow sensor had been replaced with a 90mm Pro-M and I thought that maybe the sensor may not have been calibrated properly. After the above changes and performing the usual TPS sensor adjustment and computer memory clearing, my car idles and drives much better now and the "fun factor" has been restored. Anyway, hope this helps somewhat.
