Try checking these out, and in this order:

1) Battery cables, (+ & -), grounds and hots, all cable connections to starter solenoid.

2) Check your transmission's park & neutral start enable switch.

3) Check your starter's solenoid.

Same thing happened to me, I found the problem in a loose terminal screw on the starter solenoid. I replaced the solenoid. It's now oooookay!!!!

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Jake