Battery Voltage Drop

i have E-Bike 48V 500W Motor 30A controller
i am using the battery bank of 3 parallel and 4 series batteries with 12V 7Ah battery
With full throttle battery voltages decreases from 55V to 42V but torque feel good with these batteries but rapid drop of voltages is not good

May be too much parallel and series connections causing problem
batteries connections are good

Which thing is wrong ?

Lead acid batteries? How old are they and have they been maintained, ie kept charged up?

Make each battery is fully charged 13.6 volt. I check each one individually. Look for voltage drop of one battery after some time. Check . If ok hookup. You could have one or two low capacity batteries . Plus lead is dead if you're going to pay for batteries get something that is meant to run an electric bike.
Lead needs to be fullycharged a lead battery sitting around at half charged is No good for it.