if you finally figure it is your ecm---you have a choice of buying a new unit or making the alternator a 1 wire with a trans-po conversion $35-----guys doing cummins conversions to older vehicles use these all the time---my son orders them 3 at a time when the price is right just to be sure to have them on hand when needed-----one time he ask to grab the one off my 69 gmc/cummins to get a customer on the road the same day-----if you want to see what it is--- just keyword cummins alternator regulator on ebay---they show up on amazon as well---it is the little silver box with 4 wires that fits inside the alternator case.....
Thanks for the info. Since I first posted, I found a couple videos on bypassing the computer (ECM?) with an external voltage regulator (maybe this is along the lines of your suggestion?). Also, I hooked up my multi-meter to the battery terminals and drove the truck to see if when the problem happens, would my meter also show a high voltage reading, it didn't. Stayed in the same 14v range (tried this same setup with the battery cables disconnected from battery, same result). Does this mean the battery isn't being overcharged, and that it's some other problem? BTW, this problem only happens when I'm at highway RPM, above 1500. Thanks again.