SOLVED & updated: Any mechanical/electrical genius out there

I had the same problem 2 years ago. Good voltage, good grounds even went as far as to replace the starter. The problem was a plate inside the battery seperating under load. Sometimes it would make sometimes not. Drove me crazy for 2 days while on the road until I installed a new battery.

When I turned in the old battery for a core credit, it still showed 12.7 volts. Never tried my battery boost because my battery never even dimmed the lights and never showed low voltage.