Lego RCX to NXT converter cables not working properly with EV3
These cable don't have anything connected to pins 5 or 6. This causes the software running on the EV3 that detects which type of motor is connected to think that there is nothing connected. Since it thinks nothing is connected, it periodically resets the output port which turns off the motor.
To work around this, the unregulated motor block needs to be called in a loop with two different speeds (e.g. 100 and 99) to keep the motor running. Each time the requested speed changes, it resets the device detection timer that would stop the motor.
Some people who don't mind cutting the cables have added resistors to the wires connected to pins 5 and 6 to simulate what a real EV3 motor looks like. This allows the unregulated motor block to able to be used as expected.
Here is an example of how to do that. (Although I would wire it up more like the actual EV3 motor schematic using 3.3kΩ resistors. Pin 5 to pin 4 via resistor and pin 6 to pin 4 via resistor. But if the other method is known to work...)