---

Since this came on suddenly, I'd want to know what changed, rather than just adjusting the idle.

If you take off the motor cowling, I expect you'll see that the throttle arm (that the throttle cable connects to) is not returning all the way to the idle position when you move the remote throttle to neutral - usually there are some marks to show that position, but you may need a service manual to make sure. If it is getting hung up, then you need to figure out why - could be the cable itself, or something in the motor linkage has come loose or needs lubrication.