I had the same symptoms that you describe. It was like chasing a gremlin. I could never nail it down. I replaced , fuel injectors, fuel distributor, throttle pot, OVP, fuel relay, ignition wires, and a host of other stuff I have forgotten. I have chased this devil for 2 years. One day it had the problem in my driveway. It would not start, I checked fuel and spark and it was getting both. It took a tow behind my pickup to get it going. It's a manual transmission. Once it got running it would die if the RPM dropped to less than 2500. It would not idle until it was warmed up. Took about 20 minutes before I could idle. It then drove and ran normally. Guess what it turned out to be?
The rotor and distributor cap!
High voltage was leaking & arcking all over the inside surface of the cap. Here is proof and a picture of the evidence.
http://www.davidpetryk.net/imagepages/image727.html
http://www.davidpetryk.net/imagepages/image729.html
I cleaned up the cap and things were fine for a few months. Then it started again. I finally replaced the cap and rotor and it runs like a new car. Based on the symptoms I had, I would have never guessed that was the problem.