I have the same problem on my 85 305 PU. It started fine, I would be driving and would loose spark. Sometimes I wait 10-15 min and it would start and drive fine. Sometimes an hour. No rhyme or reason to it, and it didnt happen every day. (spring and summer operation)

I swapped the module and it seemed ok for a few days then it started happening again. Then it just quit. No spark.

I first rebuilt the distributer, new Pick up coil, module and that little round capacitor, coil, cap and rotor, everything. No spark. I got Chiltons and metered it had all the correct voltages in and out, coil etc. I then borrowed a distributer from a friend and tried it and still no spark. I ran a new ground to the block, no spark.

I will check the ignition switch to make sure I am not loosing power to the distributer while starting but assuming that checks out ok, what else could it be?

i think your p/u has electronic spark control madule.
i do not remember where is located .try behind glove box or above gas pedal.
most likely esc is bad

the ESC module was all in one under the distributor cap on those years. The difference was the number of pins on one end of the module. All I have done in the past to correct this issue, is replace the ESC style module with a regular one, problem solved. Maybe this helps, maybe not, good louck.

Pull/Disconnect the four pin connector that runs from your HEI to the the firewall. Then use a jumper wire on the wire terminals A and C that lead to the HEI. Your truck should now fire and run. If not then your pick up coil at the bottom of your HEI needs to be replaced.

I have 86 GMC Vin H that had the same problem. I took it to the dealer and they showed me the trick to test the spark control by jumping the A and C terminals. It showed that my ESC was retarding my HEI to a stall. Since then I have swaped out the ESC HEI for a four pin HEI.