Considering all of the other things you have checked and/or replaced, I would tend to believe that your ECU is starting to go bad. The ECU was made with sub-par capacitors and are prone to leak over time. When this happens, the ECU gets varied voltages from the caps. and can cause all sorts of intermittant running problems. Do you have a SOHC engine, I ask because you mention the distributer??
Here is instructions for removing the ECU.
Stealth316
Around the capacitor pictured on that site, you will normally see some black, crusty looking stuff that is indicitive to leaking capacitors.
When I had my ECU's rebuilt, I used
ECM to go. They were prompt, have a test car on site and were very helpful when I called them. It will cost around $250 for a repair, providing not too much damage has been caused by the leakage. Hope this helps.