BruceO,
What are the gauges of your battery cables and ground straps?
They should be 2 gauge or thicker/larger.
1, 0 (1/0), or 00 (2/00) gauge are better.
Try running a ground strap to a starter-to-clutch-housing attachment bolt.
Other possibilities:
- The battery is weak.
- The starter needs maintenance.
- You need a tune-up (timing/carburetor, points/plugs especially)
Disconnect all cable/straps, and clean them again, make sure they are shiny, and then coat contact surfaces with copper impregnated anti-oxidant anti-sieze "grease".
Something in the system is "marginal". This is not an uncommon situation.
Good luck.
