I hate to keep mentioning this, but I just completed a rather extensive rework on the plenum side of the engine on my 88 Quad. I replaced the injectors with rebuilts with new hoses, replaced the plenum couplers, and about every hose that was on that side of the engine. One hose I repladed was dated 3 of 87. Obviously, when these cars get 20 + years on them, a lot of rubber hoses carbonize into the flexibility of hard plastic. It might be time for you to consider doing a similiar replacement of your fuel injection and vacumn hoses. I have always had a rather variable idle on this car since I aquired it last July. But now it idles like my 2010 Frontier. It is immencely more pleasureable to drive and I feel more confident about taking it out on a road trip. I am of the opinion that 20 years is about the limit on the life of rubber hoses and such.
Robert In Memphis, TN