If you haven't changed anything (like the prop) then it sounds like the idle jets....
If it's been stood a while then the fuel can go gummy in the carbs. when you power up to full throttle then the main jets are used which is a different circuit, which should run fine, but when you drop back down the revs you use the idle circuit again.
Sounds like the carbs might need a clean?
If it is the carbs then the ultimate carb clean is done in an ultrasonic bath