Bizarre electric problems, no crank/no start

v8. New Batt, new alternator. No crank/no start.  In trying to diagnose the
issue, I find that the battery terminal underhood will drop to 6volts when I try
to crank. IF I wait a minute, the volts will come back up to 12.   If I jump it off
that terminal, I can sometimes get it to start. When running, I can see the
lights dim with a voltage drop and the engine will die.  I can successfully
predict when the car will start if they shift indicator square appears around
the P.  My hunch is the IMS in the transmission is failing, sometimes not
indicating the car is in park causing a no start.  When I do get it running, I
can get intermittent ABS/TCS lights flashing and the engine will die.