I drive my Ghia for about 30 mins, engine is warm. Make a quick stop and then the engine doesn't start. It barely turns over. I wait 15 mins, still the same. I push start it and go. Then I'm stopped for almost 2 hrs, turn the key, I have no problem starting.
I suspect the weather has taken a change toward autumn where you are? The underlying problem is most likely a little corrosion here, a little corrosion there... it all adds up to a substantial amount of loss. The wiring "loop" from solenoid to (and including) started switch is the biggest culprit for robbing electrons. The fact that it is once again a problem as the weather changes is not uncommon.
Quite possibly the problem this time seems to be the same as last time, but is even more weather related. This time of year you need to remove your battery connections, clean everything up with a baking soda slurry and rinse well after the foam stops (don't get it into the cells!) Make sure the cable ends are in good condition and the inner surface where it contacts the post is round and true (and shiny bright) Same for the posts. If the cable ends are shot, replace the cables. You can treat the connections with any of a hundred different sprays and such, but I have had as good luck with LPS#1 as with anything else I've tried over the years.
Keep in mind that all those other connections are getting additional moisture, too.
Oh - you say it doesn't make sense cuz it only fails when well warmed up? Heat causes more resistance, both electrically and mechanically. The solenoid doesn't like to slam into the contacts when the piston is hot and expanded. You may want to pull your starter off, take the solenoid apart and clean it up inside, spary a little (very little) LPS1 in there, reassemble and wrap a piece of tape around the crack to keep dust and water spray out as much as possible.
I think Speedy Jim has it all on his website, BTW.
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