While trying to make a robotic arm using 2 2-wire 393 motors. We switched to them in order to be able to handle the load, but now they stall/stutter when used, even without any load attached; they will turn for a fraction of second, pause, turn, and continue like that. We have tried replacing the battery for the robot and the controller, we ensured they were going in the same direction in order to lift the arm. Do you know of any solution for this problem? Is it likely to be a problem with the motors themselves, or some electrical/wiring problem? 
Hello Newbie,
Most likely this is being caused by a load distribution issue.
Basically the Cortex has two thermal breakers, the first one is in ports 1-5 and the second one in ports 6-10. It sounds like all/most of your motors are connected into a single breaker; therefore, causing the breaker to engage giving the appearance of stalling motors.
Can you please confirm the following:
-
Color of the LEDs in your Joystick during the problem-
JOYSTICK-
ROBOT-
VEXNET-
GAME- -
Please elaborate in your motor configuration-
Port 1
Port 2
Port 3
Port 4
Port 5
Port 6
Port 7
Port 8
Port 9
Port 10
I await your confirmation.
Regards.
-Eli