Hi,
It would seem that you have checked/replaced most if not all of the usual suspects. Pls try:
1. start the engine on a dark night with the hood up. check for any arcing/high voltage spark from any of the high tension wires;
2. you may want to reconsider this step! run the engine, pull the spark plug wires (carefully, extremely high voltage) one at a time and note if the idle will not change. If no change that plug is not firing;
3. check the ignition B+12 going to the ignition coil primary terminal. If it is physically in contact with the A/C hose/line, it may be loosing a few strands inside the insulation, replace.
Other than the above, a few more checks (but not as simple):
a. check that the fuel pressure is between 50 to 66 psi;
b. replace your plugs to AC Delco CR43TSM or CR43TS or 41-803 Platinums.
This
page may be of additional informative if not an interesting read. To my knowledge, a 1995 Astro van shares the same W engine with a GMC Safari and therefore would have the same concerns/solution.
Hope this be of initial help/idea. Pls post back how things turned up or should you need additional information.
Good luck and kind regards.
Thank you for using FixYa.