In my opinion, early HEI coils failed once in a while too. I remember replacing a couple in my very early May 1974 HEI car over the years. The car would misfire occasionally under load. A new coil would fix it for a year or two. This was seat-of-the-pants diagnostics with no proof. I once had the broken wire in the pickup coil due to vacuum advance movement. The carbon buttons could fail too, but it never happened to me.
The fun one was my 1975 GMC van. Leave the engine cover loose and it would run fine. Bolt the engine cover down and the HEI module would get hot enough to act up. The engine would backfire so loud it would make your ears ring.
If modules are failing repeatedly on a newer car, one might additionally suspect heat or a bad coil. Heat issues would more often would result in a gradual degradation of performance rather than suddenly shutting down, but it could happen. 78 was a transitional year for HEI updates. I would have put the car on an ignition scope with the bad module to see what was going on, but it's easier to just put a new module in and call it a day. The original Delco modules used a very robust custom made Beryllium power transistor and most aftermarket modules used an off the shelf transistor. That's why the 990 modules seem to last longer.
Here's a link to the Delco HEI manual
http://www.pontiacpower.org/HEI.pdf
Refer to the last page for running changes Delco made in the 70's. This last page is not from Delco, but written by engineers from Bear Automotive who likely attended a Delco HEI seminar back the the day. Thankfully they took notes!
George, I remember being a high school kid and helping Mrs. Ostrega wind the field for a prototype Motorola alternator at her kitchen table.