Nope, posts are old and clean. Battery's in good shape. No corrosion anywhere. Wires look good and pass the tug test all the way down under the truck.

The reason I thought it might be the thing on the fender is that a couple of times I smelled something burning while driving the truck but chalked it up to the crappy vehicles it was near at the time. I get a solid 14+ volts when the truck is running, 12.6 when it's not. That's measured across the terminals, and also with one probe on positive and the other on the transmission cover or the valve cover.

Yesterday I ran it up to temperature, then shut it off and restarted several times to mimic conditions when it died before, but could never get it to fail. I'm starting to have a bad feeling that this is going to be one of those annoying problems that are so intermittent that it's almost impossible to diagnose.