I noticed this the other day while ruining the river. My Tach ran well past 6400 rpms when I looked down at WOT its never ran that high before normal is right around 5500 to 5600.
Now my motor didn't sound any different, I wasn't blowing out the prop because I was up on plan running normal. Was wondering why it would do this.
Mine started doing that a few months before it went out. If you look on the back you should see a small selector switch. Note the number it is on & turn it back & forth then put it back on the original number.
That would temporarily cure mine for a couple of trips before it would start acting up again.