When you have problems like this there's only one thing to do: Use the method of elimination. Change ONE item at a time until you isolate the problem.
Did you test your batteries on a good discharger that can measure internal resistance? At least measure the volt of each individual cell and see if you have a sick one.
Tried 6 cells? (a pack you know is good)
Try and change the motor also. I have an old B/L Nosram that started acting like you describe... they can be very sensitive to dirt and moisture...
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