I had a similar problem with a piece of equipment recently (not my Porsche) it would start sometimes and other times it would not. I cleaned all the connections, replaced the battery cables and solenoid....and cussed a lot.
When I put it on the charger (start mode) or jumped it from one of the trucks it would fire up every time.... Tester was showing 12+ volts.
The problem was the battery- it had plenty of voltage but not enough amps to consistently crank the engine over.
I replaced my battery and have not had a problem since.
Take it to a battery shop and they will test it for you.
2013 Porsche Boxster
Hybrid Mode
