Well, I also said that I swapped the Porsche's battery into my other car and it worked fine, then put it back in the Porsche and it still wouldn't turn the starter. The battery and cables look brand new so I don't see how it could have been a bad connection. All the more puzzling why swapping a different battery into the Porsche would have fixed the problem.

I went for a drive today and stopped and started it several times. Seemed normal.

Shrug shoulders and move on I guess...

Sounds like a bad cell in the Porsche battery...

no start
 

I also have an 87...same symptoms as I mentioned before...
you need to check the yellow ignition wire...it might be damaged between the ignition switch and its end at the engine compartment plug....it sometimes works sometimes not...

Could be. Charge it all night off car. Take it to the local car parts place and have it load tested.