Something in the ignition circuit is getting warm and then shutting down. Defective module in the distributor or bad coil? BTW, we are all as frustrated as you. We want to see you driving down the road with a big smile on your face.
This is an interesting possibility. Thank you for proposing something I hadn't thought of yet. However, the second sentence, defective distributor or bad coil might be right, but likely misguiding. If right, I have a defective new distributor. Now, the original HEI I put in the truck was one I bought from pick-n-pull, and it could have failed because it was old, or something (heat, as you mention) could be a culprit. But I can say the new HEI is definitely putting out spark. The only source of electrical is the 12v lead to the distributor, and the output from the distributor to the tachometer. Could the tachometer be backfeeding the distributor? When the truck first starts the tachometer doesn't act properly. It shows extremely high RPMs, possibly double what it should be, i.e. 4,000 rpms shows as 8,000 rpms, which is beyond the range of the tachometer. After a few minutes, it settles down. I think it's a small possibility but it's easy to test, I can simply cut the wire lead to the tachometer.
As for me driving down the road with a big smile, I'm still too nervous. At this point I worry each time I come to a stop light that something might shut down. But I sure get lots of looks!
