Have seen a couple different things cause this issue;[br]
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1. Bad voltage regulator[br]
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2. Wires under rear fender getting worn away by the tire and creating a short[br]
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3. Loose battery terminal connection[br]
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4. Speedometer might be going out[br]
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I'd start with the easy stuff like checking the wires tightening the battery connections/making sure the terminals aren't super corroded and go from there.[br]
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Hope this helps!