[bobbader]

Could be a bad battery cable or cables. Age & corrosion could be one factor. They can look OK on the outside but heavily corroded under the insulation. Diameter of the cable is another problem. Starter cable should be #1 or #2 (Not #4 or 6 as on a modern 12V car). Poor or corroded ground to the chassis can also be to blame. Every so often, It's a good idea to remove the ground cable at the chassis and wire brush or sand the area around the mounting hole on the center crossmember. Should be nice, shiny bare metal to continue to be effective.