You need to take a real good look at all of your connectors, you may have a wire thats broken inside of the insulation, happens right behind the crimp point most of the time, thats where the wire takes most of its abuse, best way to check is warm the insulation up with a lighter or hair dryer right up by the connector, then give a decent pull, if the wire is broken the insulation will stretch, another thing is to make sure the female pins in the connector are not stretched out, theres usually a flap inside the terminal that holds pressure down on the male pin when its inserted, get a safety pin and if you can slide it in with little or no resistance then you need to tighten them up, sounds like a connection problem of some sort if an old module now suddenly works, maybe you moved the harness just enough to restore the connection in a broken wire, i guess you could also try the old wiggle test while the engine is running, specifically pulling gently while you wiggle the wire. It very well could be an ignition switch problem too, but take a look at your IGN and ECM fuses, pull them out and look for corrosion or signs of heat, thats another common intermittant problem area. Other than that i would say you need to be ready to do some tests at the moment of failure next time, like check for power coming to the module and coil, check the ground between the plug on the ingnition module and the ecm and check for injection pulse, those results should help steer you in some direction from here.

 

that tach thing dimeblazer meantioned got me thinking, years ago i had a k5 where i ran a kill switch off the tach wire as someone told me grounding the tach wire would cause the engine not to start. well one day i forgot to turn my kill switch off and after cranking the truck a few time it smoked the ignition module. so if you have anything going to that tech out put on the distributor disconnect it