Your getting way too much resistance somewhere in the ground circuit.
either a broken wire inside the jacket, corroded wire and/or connectors or as Tom suggested, a dead short.
what size wires are used for the ground leads? How and where are they connected to the motor case?
Seeing as this occurs in the cranking cycle I would inspect all the wiring associated with this circuit first.
If it were me I wouldn't waste time trying to trace it down I would rip out the whole ground associated system starting at the battery then replace with 10g minimum. Once these wires get hot a time or two their life span deteriorates very quickly.
Replacing the whole ground system with fresh wire will give you a permanent fix.
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