[roadracer311]

It sounds like you've checked a lot of stuff, so I hesitate to even mention this, but if it were a motorcycle, those would be classic symptoms of a loose battery cable, at the battery. (I wrench on motorcycles for a living). Again, with the amount of wiring you've checked out, you've probably checked the terminals about 10 times, but give em a good tug if you haven't : trying to wiggle the cable side to side with your hand. It shouldn't move.