Wit them test results its a no-brainer,,the motor is shot...to make sure connect the battery direct to the motor to see if it spins full out...if so then you have a weird controller issue...If not replace the motor or the brushes
Could be but in my experience a motor with bad brushes,,commutator or armature will hot soak batteries pretty quick and show a decline similar to dying batteries but with the couple years you have on it I wouldnt doubt if you may have to replace both..........................I will try that. However, im starting to think it is the batteries. I put the multimeter on the batts and it read 39, gave it full throttle and the voltage dropped immediately to 29v. Rechecked with no throttle and voltage was down to 30v. I only ran it for 15-20 seconds with the back tire off the ground.