I read all the way thru twice and was about to admonish you for not putting in NEW battery cables. 25 year old cables are too old. Cleaning the ends does not help internal, unseen corrosion, breaks in cable strands, etc. No such thing as "heavy duty" cables. There are large gauge cables and small gauge cables and the correct gauge cable.........then at the very end of all this you suddenly said "new battery cables". Just want to make sure you meant that because that was my no. 1 guess as to all this "trouble". As mentioned by an astute poster, heat breaks down conductivity. You also need to get rid of the regulator. They are not worth messing with in todays world. Can't imagine you went thru most of this with 25 year old cables. Also as a rule of thumb: mechanics don't say: "it's been working for 25 years so it must be OK".
It seems to me that after all this testing and load testing and regulator cleaning and cable clamp cleaning, you have always had enough battery voltage to start a motor. It seems to me that even when your truck was jump started it still slowly turned over with a supposed new starter and battery of your own, that it's obviously the cables. (if it's not a mechanical/seizing/valve train issue/flooding).
What I ALWAYS say is that it is ALWAYS the battery, but you say it's new and shows no breakdown with a non professional load tester. If you really did get BOTH new cables with factory ends and the ground is on shinny clean motor metal, then call me crazy.
