This is a common problem with early Porsche 928's. They overheat the starter motor and the fix is to fit an extra relay. Note that when hot all you get is a click!

The overheating starter motor was very inconsistent and very annoying and more than one
mechanic told me to cool it with cold water. Like Murphy's Law it always happened at the
most inopportune time. Hence, a bottle of water always on hand. It seems it wasn't just
a 'Toyota' thing. Thanks Ramona.