Unusual RPM when warm

Hello hope someone might give me some ideas.

I have a 2003 20hp 2 stroke yamaha. It's been a great motor and has never put a foot wrong.

Until last time I took it out. It was fine at first but when it got up to temperature it started acting strangely.

When you increase the throttle it suddenly jumps to a high revs. The same when you decrease, it goes from a high revs then suddenly jumps to going slow.

It's like it can either go fast or slow, but not in between.

I can't find anyone who knows.

It's not the cables, because it is the same when i push the throttle by hand. The throttle mechanism seems ok, nothing obviously wrong.

Help!

Is then happening when in gear or when in neutral? If it is happening when in gear, does the boat accelerate quickly or do the revs just rise? If so, there is a chance that the prop has spun on the shaft and you will need to get a new bushing pressed into it.

Stuart