BJ -
This is REALLY basic..... But..... If yo haven't already.....
Take the cables off both battery terminals.
Check the terminals and terminal ends for corrosion.
Use a battery terminal brush to clean them well - even if just for preventative maintenance.
Be sure you tighten them back up good and snug; not over-tightened.
If the ends are loose, not only do they corrode much more easily, I have seen the battery terminals actually melt from the heat of the starter current draw.
If the battery terminal ends are the type that just clamp to the wire, be sure they are secure and free of corrosion where they meet the wire.
If they are corroded, auto parts stores carry replacement terminal ends. Cut the end off the old wire (you'll need to get a BIG pair of wire clippers to do it right), strip about 3/4 inch to expose new wire and clamp on the new ends.
Have you taken the cranking battery to a parts store or someplace like Wallyworld so that they can load test it and make sure the battery is good / doesn't have an internal short??
me!