I had the same sequence of experiences, Carl
I had switched to 12v (in the late '60s) because:
- everyone said to do it
- starting/cranking was slow (and getting slower)
Many years later (early this century) after a total restoration:
- cleaned all grounds
- installed correct 6v size battery cables and ground straps (0 or 00 cables & straps)
- installed a Runtz-style voltage reducer into the fuel gauge
- got the starter rebuilt by a pro
That was over 15 years ago.
The only time that I have a problem starting is at night when I forget which direction to turn the ignition switch.