Sounds like you have some poor connections, maybe it is time to remove, clean and make sure all the connection are good, especially on the grounds
I use two # 2 cable grounds in the trunk off of the battery and multiple #4 grounds from the frame to the motor also
you need to keep in mind that all current, voltage and amperage, comes out of the negative side of ALL batteries and is returned through the positive side
The only time current flows form positive to negative is in the battery