[Joe Wicked]

John,
If it is not the alternator, then it has either got to be a ground issue or a loose contact somewhere. Remember it could be a loose contact or ground issue inside a component like the MSD, Alternator, or any other electronic component. There also could be a problem with the battery itself causing it to load down the system and pull the voltage down. Normally though I would expect that to be the inverse of what you saw, cooler morning would be fine but hotter temps would cause it to be worse. If possible next time you see it, start pulling fuses to components that are not essential to keep it running. You may get lucky and find it is a fan or something. Worse case you narrow the field on what it can be by ruling out the peripherals. If you have a schematic for the car they can be very helpful.