Is your payload correct ? Did you use payload identification function or is it manually entered ? Do the inertias entered reflect the real inertias of the tool ?
You allready ruled out the amplifier so the next 2 possibilities of the fault are bad J2 motor and/or bad J2 reducer.
Do you have collision guard option installed ? if so, in which sensitivity ? The fact that robot performs ok in auto mode (where the movement is fluid I guess without excessive starts and stops) suggests that the torque required to move J2 from a halted state is greater than the amount allowed from collision monitor and that's why it throws the error. This suggests either a mechanical issue or a faulty motor.
Put the robot in auto mode and use the status > Axis screen to observe the Torque Monitor and the Disturbance monitor. Watch out for the J2 Axis performance. If there is a problem with the motor or the reducer the numbers will speak for themselves.