grounding

Ground wires should be a triangle, like;

Battery to body
body to engine
engine to battery

If any of the wires are aftermarklet/replacement/home made, perhaps a thicker gauge would help and on all 3 wires. Clean clean contact area tightly snuggged down.

Also, the starter could be taking on a bit of heat. Headers? Heat sheild!
Battery OK? Overcharging? Cells filled with fluid?