[dmjlambert]

Well if your new DUI distributor starts getting failed modules, I would next suspect the alternator and aim to put a 3 wire internally regulated one on the truck, with the sense wire connected at a junction away from the alternator (not just connected on the big post of the alternator with a few inches of wire). I have also seen really weird things happen in vehicles due to a battery that is failing but still cranks and works in the vehicle for a couple more years. I have not seen distributor module failure due to that, but it would not surprise me. I do wonder about your short duration voltage drops to 2 or 4 volts, and about the dropping to 9v during cranking. Do you have electric fans on the radiator?