[FrankJ]

Both motors depend on fan cooling. At zero speed you do not get any. Also the higher the voltage, the higher the stall current & the more heat from I^2R losses. So at 12 volts you rapidly cook the motor. At a lower voltage it will take longer. Running a PID loop you are running just enough voltage to hold position. Manually you are probably running more voltage (& current) than you need.

We has a similar issue with our Logomotion arm. We solve it by limiting the joy stick command to just enough to stall the arm. We had a turbo button to increase the command to actually raise the arm when needed.