Uh oh...voltage crash and now won't turn over

Hi Guys,
The other day I was out as a cruise night and noticed my voltage all of a sudden became erratic on the low side at idle. I was actually idling through the parking lot when the voltage dipped and caused a stumble and stall. At increased rpm the voltage recovered and all was good. Checked connections...all good. Got her home and put her to bed. Last night when heading out for test drive she ran ok but the voltage was a bit low @ 11 volts...always runs a solid 13. Again checked all connections, Alternator connections as well, changed vr because I just happened to have a new in the box one in the cabinet. All grounds are good. Battery voltage is 12.4. Now the starter won't even crank....like it's not getting any power at all. Ignition key to on and voltage gage @ 12 volts, key to start and voltage crash to 10 volts or maybe lower as that's the bottom range of the gage. Everything works with the key in the on position...lights etc. Fusible link appears good. HEI conversion for over 2 years. Car was running awesome prior to the other day.
Thoughts?....you guys are the best by the way.