2001 6CTA wont return to idle

Have a 2001 Searay 420 AC with a pair of 6CTA’s.  After running up to cruise at 2150 the port motor will not return to idle.  It will stay at 1000 rpm.  shift to neutral shut down and restart engine will return to idle.  If you bring it up past about 1300-1500 it will again not return to idle.  All Mechanical controls.  Glendinning syncronizer is configured with Starboard motor as slave, though I suspect the syncro is irrelevant in this situation. Thoughts?