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I have been a professional master mechanic for over 35 years and am happy to help you with this.
This is easy enough to test if you have a volt meter or test light.
When its in a NO START condition, you need to check for battery voltage at the small black wire on the starter solenoid located on top of the starter.
If it has power when someone is trying to start it, then it can only be the starter armeture has a "hot spot" on it and needs replaced.
If it does NOT have power to that small black wire when someone is trying to start it, then the starter relay needs replaced.
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