I didn't see any mention of you measuring the injector resistances. It is very easy to do and would quickly tell you if you have injector coils going bad, you don't have to pull the plenum to check them. Here is the link to an article on Marc Haibeck's website about measuring the injector resistance:
http://www.zr1specialist.com/HAT%20W...Resistance.htm
The diagram on his site is for a '91 though it would be much easier to read in the shop manual if you have one.