Well, it sounds like you guys have Fumblin46 on the right track. I just wanted to throw in my two cents worth.
I'd go Glenn one step further and say that his first law should be a commandment
Virtually every time I have worked on an electrical issue on my old trucks over the years the final resolution has usually been a grounding problem. Just chant the mantra, "ground good, good ground" over and over again as you work on diagnosing and solving any electrical problem.
On my 3600 (I know, it's not a "big bolt", but it's definitely a "wannabe" :p ) I have a heavy duty grounding strap that goes from the neg terminal of the battery to a point on the frame near the starter. Before attaching, I ground all paint and rust off to get a nice shiny bare metal spot. From the same attachment point, I ran a grounding strap to the starter bolt to ground the engine.
