Given that it's an sp300, i'd lean towards the cut carriage cable. It carries the power for the cut solenoid, the crop mark sensor, and pinch roller sensors. Usually it's not an on/off issue with that cable, more intermittant. You could test it with the sensor check in service mode with some white vinyl loaded, move the cut carriage over the vinyl and grab a black piece of vinyl or
paper and wave it under where the sensor is. you should see on the screen one of the letters has an asterisk under it that turns on and off as the black passes under the sensor. With the black beneath it, move the carriage along from one side to the other, and see if that asterisk flickers, since it shouldn't if the cable maintains the signal.