Swap the left and right to see if it follows. Twist the cable as you re-insert it at each end so it wipes any light oxides away.
Your receiver could be the source of the problem, too. Turn it OFF and manually operate all of it's controls and switches throughout their ranges, especially those that are rarely used, like Tape Monitor or Mute. Built up internal resistance might be the problem.
Try connecting the CD player to another input (not PHONO).
Answers & Comments
I'd suspect a crappy cable connection.
Swap the left and right to see if it follows. Twist the cable as you re-insert it at each end so it wipes any light oxides away.
Your receiver could be the source of the problem, too. Turn it OFF and manually operate all of it's controls and switches throughout their ranges, especially those that are rarely used, like Tape Monitor or Mute. Built up internal resistance might be the problem.
Try connecting the CD player to another input (not PHONO).