Assuming the clutch is adjusted right and you have a proper motor oil (wet clutch approved) in it and the oil is clean, more than likely that it is caused by one of four things:
1) Weak or failing battery
2) Dirty/corroded or wet harness connector(s), or dirty/corroded/loose battery & grounds (motor & frame) cables.
3) Poorly lubed reduction gears and/or reduction gear bearings are cocked in the housing/case & need fresh lube. Or the housing bolts were overtightened and caused warpage/binding of the shafts/gears.
4) Shift motor needs to be taken apart for cleaning, fresh grease, or replaced if water has gotten inside or bearings, brushes are bad.
Of course, if there are any china parts put in the system in place of OEM, that becomes the number one reason.
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1) Weak or failing battery
2) Dirty/corroded or wet harness connector(s), or dirty/corroded/loose battery & grounds (motor & frame) cables.
3) Poorly lubed reduction gears and/or reduction gear bearings are cocked in the housing/case & need fresh lube. Or the housing bolts were overtightened and caused warpage/binding of the shafts/gears.
4) Shift motor needs to be taken apart for cleaning, fresh grease, or replaced if water has gotten inside or bearings, brushes are bad.
Of course, if there are any china parts put in the system in place of OEM, that becomes the number one reason.
Welcome to the forums!