This is often caused by the control cable starting to seize up, or the control lever assembly needing a friction adjustment.
If you have Volvo Penta
controls as on my Lagoon
410 with Yanmars, the friction adjustment is easy. My memory is a bit hazy but you prise the cover off and a small adjuster screw is clearly visible and obvious - one for each control (allen key type I think). You may need to remove the handles.
There has to be enough friction to stop it slipping back to idle.
If that does not resolve it, then the cable is the most likely culprit. Remove from each end and see if it slides easily. If not, they ae normally just replaced rather than trying to free them up. They are a common item and readily available in the length you need. I paid about $50 2 years ago.
If replacing the cable, it's a good idea to pull a line through with the end, so you can pull the new cable back the same way.