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First thing to check is, the starter.
They do not last forever, especially w/ headers + hot climates.
A starter motor in the throes of death probably starts cranking slower and slower.
Until, it totally dies.
IF vehicle battery's strong, check starter.
Which may involve removal for testing.
check your battery connections, then alternator connections then starter connections. Then test the battery/starter/alternator circuit. youll find the issue.