I have a 48 volt system with a nominal 500 watt rear hub motor. The voltage showing on the controllers lcd screen will be showing 46 volts or even 49 volts when the bike will suddenly lose power. I can stop and turn the battery on off switch off then back on and it will start going again but will stop again under mild load. The batteries in the 13s pack show 3.9 volts evenly in all cell groups. I am contemplating replacing the bms but what do you think?

I think you might do better trying the "Electric Bikes" forum here.