We suggest contacting your dealer to have the cable checked out and if defective replaced. Sometimes the jacket that protects the cable actually catches causing this type of issue. This can be remedied by trimming the jacket to avoid catching.
FYI, the engine speed should always be set to full throttle when ever the mower is in use. Use the variable speed control selector located in the center of the control console to modify ground speed, but you always want full engine RPM to the deck and hydraulics.
-The Toro Company