BBC starts fine when it is cold. When it reaches operating temp it will not start. Won't even crank over, no click, nothing at all. At that point battery reads 12.5 volts, put a set of jumper cables on it from another vehicle and it fires right up. I have tried two different starters with new sol. put a heat sheild on the sol. a new battery, I have replaced the horn relay, ignition switch and checked the neutral safety switch.
The alterantor has been upgraded to internal voltage regulator, it is charging 14.6 volts at the rear of the alternator and 14.4 at the battery.
When you jump start the vehicle, it takes 2 seconds to start.
Would appreciate any advice you have to offer.
Thanks,
Scott

try going straight to the solenoid with a remote starter switch to eliminate swith and circuit.check votage drop between starter housing and batt neg and then batt pos to sol when cranking. should be no more than .02 volts

You could have a bad cable or a missing ground.You covered just about everything else.

Scott, my 68 had the same problem that got progressively worse over time. I had excessive voltage drop measured on the large purple starter solenoid wire.

What you might do is rig a wire up to the starter solenoid post for that purple wire and just leave it on. Wait until the next time the starter goes limp on you when you try to crank, and then measure the voltage while you have the key in the crank position(you must test the circuit under load). In my case I only had about 6 volts, which was very marginal and barely enough to work the solenoid. As things heat up, it's no longer enough power. I bet you have the same problem because this circuit is long and stupid with lots of connection points that can degrade over time. A lot of people have this problem.

Do the test and post back. We can go into more detail from there.

Thanks Chris, just getting back from vacation. I will try your suggestion.
Scott

Try this it works for me.
http://www.longislandchevelles.com/starter_mod.html

Hey Guys,
Thanks for all the information. It ended up being a bad positive bat. cable. Also was a missing engine ground to chasis strap.
Scott