weird electrical gremlin in my bronco

[C-dub]

I just got my truck back from ford, they replaced a faulty sending unit they installed about 1 month ago (free of charge). on they way home my volt meter gauge drops to about 10volts (at the n in "normal"). I get the truck home thinking loose battery cable, every thing looked good, so I cleaned them up and started up the truck. volts back up to normal. drive the truck to work the next day and the volts drop again (wtf) I did the old pull the batt cable trick and sure enough the truck dies. alternator was replaced about 6months ago. thinking it was weird I start the truck again and the **** volts go back to normal (wt. x 2) pull the cable again and truck runs fine. I brought it back to ford today for them to look it over as the truck was fine when I brought it to them... at least the charging system. they called me and asked me to tell them what the symptoms are...again and if it had any thing to do with the blown fuse that was in the cup holder (wtf x3) the fuse is for the machs cigarette lighter and way to small for any thing in a 96 bronco. it dawned on me this afternoon that the alternator is fine when the truck is cold, but after it runs for about 15-20 minutes the alternator cuts out. any ideas what this gremlin can be?