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Make and model? Voltage? Do you have a voltmeter to check each battery's voltage?
1999 Ezgo, 36 volts!
How old are the batteries?Any one weak battery or a bad connection will affect the whole system.
I would use voltmeter and check the voltage while driving , to see what happens to the battery voltage. With it slowing on hills , voltage has to be dropping!
I see question is still open, did you get to check the batteries?